UV Photofragmentation and IR Spectroscopy of Cold, G-Type Î²-O-4

Article pubs.acs.org/JPCA

UV Photofragmentation and IR Spectroscopy of Cold, G‑Type β‑O‑4 and β−β Dilignol−Alkali Metal Complexes: Structure and LinkageDependent Photofragmentation Jacob C. Dean,† Nicole L. Burke, John R. Hopkins, James G. Redwine,‡ P. V. Ramachandran, Scott A. McLuckey,* and Timothy S. Zwier* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States S Supporting Information *

ABSTRACT: Ultraviolet photofragmentation spectroscopy and infrared spectroscopy were performed on two prototypical guaiacyl (G)-type dilignols containing β-O-4 and β−β linkages, complexed with either lithium or sodium cations. The complexes were generated by nanoelectrospray ionization, introduced into a multistage mass spectrometer, and subsequently cooled in a 22-pole cold ion trap to T ≈ 10 K. A combination of UV photofragment spectroscopy and IR-UV double resonance spectroscopy was used to characterize the preferred mode of binding of the alkali metal cations and the structural changes so induced. Based on a combination of spectral evidence provided by the UV and IR spectra, the Li+ and Na+ cations are deduced to preferably bind to both dilignols via their linkages, which constitute unique, oxygen-rich binding pockets for the cations. The UV spectra reflect this binding motif in their extensive Franck−Condon activity involving low-frequency puckering motions of the linkages in response to electronic excitation. In the pinoresinol•Li+/Na+ complexes involving the β−β linkage, the spectra also showed an inherent spectral broadening. The photofragment mass spectra are unique for each dilignol•Li+/Na+ complex, many of which are also complementary to those produced by collision-induced dissociation (CID), indicating the presence of unique excited state processes that direct the fragmentation. These results suggest the potential for site-selective fragmentation and for uncovering fragmentation pathways only accessed by resonant UV excitation of cold lignin ions.

I. INTRODUCTION Lignin is ubiquitous in plants and an essential component of plant cells, accounting for 15−30% of the biomass in the cell wall. Lignin provides structural support and rigidity to the plant cell wall1−3 and surrounds and protects the cell wall polysaccharides (cellulose and hemicelluloses) from pathogenic degradation.1,4−9 In biofuel production, lignin’s high structural integrity presents a challenge to efficient and controlled degradation of lignin for the harvesting of biofuel from lignocellulosic biomass.10,11 As such, significant effort has been directed toward increasing our molecular-scale understanding of the plant-specific and site-specific properties of lignin with the goal of achieving better control over the structural properties and degradation pathways of lignin.10,11 The classification of lignin is predominantly made by specifying the relative abundances of the three aromatic ring types (H, G, S) incorporated into the polymer, derived from the three dominant monomers of which it is composed: pcoumaryl alcohol (H, hydroxyphenyl), coniferyl alcohol (G, guaiacyl), and sinapyl alcohol (S, syringyl). The relative abundances of these monomers vary with plant species1,9 and cell type5 but are also influenced to some degree by environmental factors.3,12 Characterization of lignin structure © 2015 American Chemical Society

is complicated by the lack of specificity in the sequence of monomers, as the polymerization occurs by a nonspecific radical coupling mechanism.1,9,13 As a result, the relative abundances of the H, G, and S subunits modulate the degree and type of cross-branching in the biopolymer structure. Second, the various linkages that separate monomer subunits manipulate the structure of the polymer at a molecular level by introducing diversity in interunit flexibility and incorporating different hydrogen bonding donor/acceptor arrangements. The variety of linkages expands the permutations of characteristic “units” in the polymer, introducing further complexity to the structure while also raising the prospects for a more rigorous fingerprinting mechanism for characterization. The primary radical coupling site is at the β-position of the double bond in the monolignols, yielding three primary β-dilignol linkages defined by the terminal positions they link: β-O-4, β−β, and β5.9,14 These dilignol linkages connecting two guaicyl units are shown in Figure 1. The β-O-4 linkage is the most prevalent, accounting for 45−60% of the linkages in lignin and marking Received: December 17, 2014 Revised: February 3, 2015 Published: February 19, 2015 1917

DOI: 10.1021/jp512603n J. Phys. Chem. A 2015, 119, 1917−1932

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The Journal of Physical Chemistry A

for unprecedented control over the placement of energy into the ion for site-specific fragmentation, if the initial site of excitation influences the fragmentation pathways. As a fundamental first step toward this longer-term goal, we present here a combination of IR and UV spectroscopy that determines the structures of two of the most prevalent guaiacylbased dilignols, G(β-O-4)G and G(β−β)G, complexed to Li+ and Na+ cations. The β-5 linkage, which is more difficult to synthize, is left for future work. The present study builds on earlier work by our group on prototypical lignin chromophores as neutrals. The cation complexes were chosen over the dilignol anions to avoid undesired photodetachment following UV absorption. Using a “bottom-up” approach similar to the MSnbased lignomics studies, we have previously characterized the conformation-specific IR and UV spectroscopy of the monolignols themselves,25 a set of simpler H/G/S aromatic subunits,26−29 a monomer derivative that incorporated a β-O-4 side-chain,30 and the G-type β-O-4 and β−β dilignols.31 Such studies are necessary for interpreting spectra of larger oligomers where the spectra are anticipated to be increasingly complex as the number of IR and UV chromophores grows. In extending our work to the alkali metal cation complexes of these same two G-type dilignols (erythro β-O-4 and (±) β−β pinoresinol), we use a combination of UV and IR-UV double resonance to determine the primary binding site of the alkali metal cations to the dilignols and the influence such binding has on their spectra relative to the neutrals. Then, armed with a knowledge of these starting structures, photofragment mass spectra are studied to characterize their specific fragmentation pathways. As described below, pathways shared by CID and unique to the UV excitation process are discovered, pointing the way for future studies on larger lignin oligomer ions.

Figure 1. Chemical structures for the primary G-type β-dilignol linkages.

this linkage as a key contributor in shaping the potential energy landscape of lignin.15,16 The emerging field of “lignomics” has concentrated efforts on elucidating lignin structure by “sequencing” oligomers (oligolignols) into their assorted unit types and linkages.2,7 While NMR methods have been extraordinarily fruitful in characterizing the lignin structure, their strength is in determining the relative abundances of each type of monomer and linkage in the structure but does not directly address sequence-specific properties.9,17 Tandem mass-spectrometry (MSn) is the principal tool for achieving this latter goal, implemented by characterizing “marker” fragmentation pathways generated from collision-induced dissociation (CID) of model dilignol ions and applying those results to the interpretation of fragmentation mass spectra of larger oligomers.2,7,18,19 These studies have predominantly focused on negative ions due to their facile formation via electrospray by deprotonation of the phenol OH.2,7,18−20 Positive-mode electrospray ionization (ESI) of lignin analytes was found to be most effective when binding an alkali metal cation to the analyte, relative to the much less efficient protonation of lignins in ESI.19 The MSn approach is showing promise in both the evaluation/ classification of oligolignol samples and in assessing the efficiency of fragmentation at specific sites along the chain via the CID technique. However, given the inherent UV absorptions of the monolignols, resonant UV excitation provides an alternative mechanism for imparting the necessary energy for fragmentation to the lignin ion, motivating the present studies. The present paper focuses its attention on UV-induced photofragmentation studies of prototypical dilignols using a recently constructed multistage mass spectrometer with a cryocooled photodissociation region.21 By mass-selecting a given parent ion and then cooling the ions down to ∼10 K, the Boltzmann population is funneled into the zero-point levels of the most stable conformations, and vibrationally resolved UV excitation spectra can be recorded for those stable conformational isomers. UV excitation of these cryocooled ions offers a powerful means for imparting well-defined amounts of energy to the oligolignol ion. Subsequent fragmentation then proceeds by nonradiative relaxation to the hot ground state or by excited state transfer to dissociative electronic states.22 The former mechanism generally produces fragment ions analogous to CID, while the latter has been shown in other contexts to generate fragmentation pathways only accessed by the excited electronic state.22−24 Furthermore, since lignin by its nature is composed of an array of similar, but distinct, aromatic moieties (H, G, S) connected by unique linkages (β-O-4, β−β, β-5) that perturb the electronic spectrum, selective excitation of specific sites along the oligolignol backbone may be possible. Thus, UV excitation of cryocooled, mass-selected ions holds the potential

II. METHODS A. Experimental Methods. The β-O-4 sample (guaiacylglycerol-β-guaiacyl ether) was purchased from TCI America in stereochemically pure (S,R) (erythro) form, as determined by polarimetric analysis ([α]D ∼ +9). The (±)-pinoresinol (4,4′(hexahydrofuro[3,4-c]-furan-1,4-diyl)bis(2-methoxyphenol)) sample was synthesized by the procedure used in Roy et al.32 The (+) and (−) isomers of pinoresinol are spectroscopically and conformationally equivalent, as the erythro designations of (S,R) and (R,S) diastereomers are indistinguishable. Given this fact, pinoresinol is treated as a single stereochemical species for the spectroscopic analysis to follow. The multistage mass spectrometer used in this experiment has been discussed in detail elsewhere,21,33 but a brief description will be given here. The sample solutions were prepared in a 70:30 methanol:water solvent mixture at a concentration of ∼1−2 mM, with LiCl or NaCl added at a similar concentration. The dilignol-cation complexes were brought into the mass spectrometer by nanoelectrospray ionization (nESI), and the parent ion was trapped and isolated in a linear ion trap (LIT) via a notched chirp broadband sweep. The mass-selected parent ions were then turned down the coldtrap axis by a quadrupole ion deflector and trapped in the 22pole cold ion trap prefilled with ultrapure helium. The 22-pole ion trap is held at 5 K by a closed cycle helium cryostat (Sumitomo Heavy Industries). The ions are cooled sympathetically by collisions with the thermalized helium (THe ≈ 5 K) for approximately 25 ms prior to UV excitation. UV photofragmentation (UVP) spectroscopy is carried out by scanning the frequency-doubled output of a Nd:YAG 1918


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The Journal of Physical Chemistry A (second harmonic) pumped dye laser (UV energy ∼1.5 mJ/ pulse). When resonant with a UV transition, the molecule− cation complex is excited and fragmented via a nonradiative internal conversion pathway or by excited state transfer to a dissociative surface.22 The fragment and remaining parent ions are then ejected from the 22-pole ion trap back through the entrance lens and turned by a quadrupole ion deflector into a second LIT where they are trapped. After collecting the ions in the final LIT, a broadband waveform is applied (33220A Agilent Technologies) to remove the remaining parent ions, and the fragment ion population is then dumped into a MS channeltron electron multiplier detector (4773G, Photonis USA).34 This detection method removes the mass analysis step during the spectroscopy experiment but enables the detection of a whole range of fragment product masses simultaneously. As a result, the relative intensities of different conformational isomers in the resulting UV spectra are more accurately represented as the sum of all fragmentation channels are captured at once. For mass analysis of the photofragments following UV excitation, the photofragment ions are isolated and analyzed by mass selective axial ejection (MSAE)35 in the LIT. IR-UV double-resonance spectroscopy was performed by spatially overlapping the UV photofragmentation laser with a Nd:YAG pumped infrared parametric converter (LaserVision) in a counter-propagating fashion. The IR pulse is introduced to the cold trap 100−200 ns prior to the arrival of the UV pulse. The IR-UV double-resonance spectroscopy was carried out in two complementary ways: (1) photofragment depletion mode,22,23 termed in the following sections as infrared fragment ion-dip spectroscopy (IRFIDS), and (2) photofragment gain mode, coined IR fragment ion-gain spectroscopy (IRFIGS). The spatial and temporal setup of the UV and IR lasers remains the same between the two experiments; the only experimental difference is in the UV laser wavelength used as probe. For IRFIDS, the UV laser is fixed on a vibronic transition of one of the conformers present in the cold trap, and when the IR laser is resonant with an IR transition of the same conformer (shares the same zero-point level), depletion of the UV-induced fragment ion signal is observed. IRFIGS on the other hand is accomplished by fixing the UV laser wavelength ∼2 nm red of the longest wavelength origin transition. In this case, when the IR photon is resonant with any conformer populated in the ion trap, a gain in fragment ion signal is observed. This method is analogous to infrared ion-gain (IRIG) spectroscopy in neutral molecules,30,36 where the UV absorption of the IR-excited molecule, with its increased internal energy, is broadened and extended out to the probed wavelength thereby producing IRinduced gains in resonant UV-mediated ionization signal. In UV photofragmentation the same scenario exists, except that an increase in photodissociation efficiency induced by the higher internal energy of the excited ion can also accompany the process, which can further enhance the gain in fragmentation yield at the probe UV laser wavelength. B. Computational Methods. To identify the possible conformational minima and metal-cation binding conformations for each molecule−ion complex, a conformational search was performed using the Amber* force field37 in the MacroModel suite.38 Approximately 100 of the lowest energy structures for each complex were further optimized by density functional theory (DFT) using the hybrid M05-2X functional39 with the 6-31+G(d) basis set. The geometry optimizations were performed with a tight convergence criterion and an ultrafine

grid. After geometry optimizations, harmonic vibrational frequency calculations were carried out for comparison with experiment. Vertical excitation energies were also calculated with time-dependent DFT (TDDFT) for comparison of the S0S1 and S0-S2 transitions among structures. All of the DFT calculations were performed using Gaussian 09.40 The calculated harmonic OH stretch frequencies were scaled by 0.9495 and 0.9535 for the guaiacyl OH and linkage OH stretch bands respectively for comparison with experimental spectra, scale factors established in previous work to account for anharmonicity.30

III. RESULTS AND ANALYSIS In order to generate lignin cations by ESI, complexation with Na+ or smaller Li+ ions has been shown by Kenttämaa and coworkers to be much more efficient than protonation.19 As such, the following sections detail the results of UV photofragmentation spectroscopy and IR-UV double-resonance spectroscopy of the β-O-4•Li+, β-O-4•Na+, pinoresinol•Li+, and pinoresinol•Na+ complexes. A. Computational Prediction of Conformational Families. Several structural designations exist for β-O-4 and pinoresinol complexes that are used to distinguish different conformations. The structural labeling scheme is laid out in Figure 1. First, as in the case for neutral β-O-4,31 the two aromatic rings are not equivalent and are therefore designated ring 1 (R1) for the guaiacyl ring and ring 2 (R2) for the omethoxy aryl moiety. The linkage atoms are labeled α, β, and γ going from R1 → R2, and the OH groups are then labeled α− OH and γ−OH. The rings in pinoresinol are equivalent and no unique OH groups are present in its linkage; therefore no specific distinctions need to be made. The carbon atoms making up the β−β linkage are labeled α, β, and γ as shown in Figure 1. In the case of lignin-ion complexes, the primary driving force dictating the structures formed is the binding of the Li+ or Na+. For the case of β-O-4, two primary binding motifs were found for the Li+ complex among the relevant calculations: (1) an extended trans backbone structure (CφR1CαCβO = 175°) with the α−OH, γ−OH and two R2 oxygens forming an electronrich pocket for the Li+/Na+ docking, or (2) a folded “sandwich” cis backbone structure (CφR1CαCβO = 21°) which places the R1/R2 planes parallel with one another allowing the cation to bind to the γ−OH, R2 oxygens, and the π-cloud of R1. These are distinguished as “type 1” (1) and “type 2” (2) respectively, and structural examples are given in the Supporting Information. Within these families, the directionality of the asymmetric R1 ring generates syn and anti minima corresponding to the alignment of the OCH3 with the α−OH group. For all low energy structures, the R1 OH•••OCH3 H-bond is retained and unchanged between structures. Finally, more subtle differences can be found among some minima through the dihedral angles associated with torsion about the Cβ-Oβ and CφR1-Cα bonds, which are discussed as necessary. The structural designations used for these families henceforth will be (1,2)/ syn,anti for the (binding type)/α−OH/OCH3 orientation, with further distinctions between those families explicitly given. According to the calculations, the primary binding site found for pinoresinol was in the center of the linkage and anchored by the ether oxygen atoms. This configuration dominated the low energy structures both energetically and in total number. From there, the chief conformational freedom is in the internal rotation of the Cφ-Cα bond orienting individual rings syn, anti, 1919


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Figure 2. UVP spectrum of (a) β-O-4•Li+ and (b) β-O-4•Na+ with scaled TDDFT vertical excitations given as sticks below. Asterisks indicate transitions where mass analysis of the photofragments was performed.

or perpendicular (perp) to the nearest ether oxygen defined by the asymmetric guaiacyl OH•••OCH3 direction. Given the two equivalent local environments of the rings in pinoresinol, the rings also natively arrange either syn or anti to one another. The structural designation used throughout the rest of this work is therefore syn,anti,perp/syn,anti,perp(syn,anti) for ring 1 direction/ring 2 direction (relative ring orientation). B. β-O-4•Li+/Na+ Spectroscopy. i. UVP Spectroscopy and Photofragment Mass Spectrometry. In analyzing the UV spectra of these complexes, it is instructive to compare with the results of the neutral dilignols, and an active comparison will accompany the analysis that follows.31 The UV photofragmentation spectra of β-O-4•Li+ and β-O-4•Na+ are shown in Figure 2a and 2b, respectively. The UV spectra display dense vibronic structure that extends well over 1500 cm−1 above the first observable origin transition, which appears near 35000 cm−1 for both complexes. The lowest frequency origin transition in the Li+(Na+) complex is 421(399) cm−1 red of the major conformer observed in the neutral molecule, reflecting a significant effect of the metal ion on the electronic transition. Upon inspection of the low frequency region of the UVP spectrum shown in close-up in Figure 3a, three origin-like transitions are apparent in the β-O-4•Li+ spectrum, identified by their large intensity compared with the low-frequency vibronic transitions built off them. These bands are located at 34985, 35083, and 35161 cm−1, and are tentatively assigned to the S0-S1 origin transitions of three conformers, labeled A, B, and C respectively. The high UV pulse powers used in this experiment serve to partially saturate the vibronic bands built off of these origin transitions, leading to their respective origins appearing as “terraces” in the spectrum. This partly carries over into the β-O-4•Na+ spectrum, with two origins clearly separated by 50 cm−1 and labeled conformers A and B. In order to identify with certainty these transitions as arising from distinct conformational isomers, holeburning spectroscopy is needed. However, in the present case, as we shall see, the IR spectra are nearly identical, preventing a clear distinction, but proving the shared binding motif (section III.B.ii). In the case of the neutral β-O-4 dilignol,31 two conformers were observed, both characterized by a linkage conformation that incorporates an α−OH H-bond with the OCH3 group on R2, and a weaker γ−OH H-bond to the R2 ether oxygen. The two conformers differed only by a 180° internal rotation about

Figure 3. (a) UVP spectrum of β-O-4•Li+ (top) and β-O-4•Na+ (bottom) in the low-frequency region of the S0-S1 transition, with scaled TDDFT vertical excitations of the three lowest energy conformers given as sticks below. (b) Photofragment mass spectra of β-O-4•Li+ (top) and β-O-4•Na+ (bottom) taken while exciting the conformer C and B S0-S1 origin transitions, respectively.

the R1 Cφ-Cα bond, yielding a major syn conformer and a minor anti conformer with respect to the α−OH/OCH3 configuration. This difference in R1 position was primarily manifested in the splitting between the origin transitions (243 1920


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The Journal of Physical Chemistry A cm−1) with the anti conformer appearing at lower energy. As we shall see for β-O-4•Li+/Na+ however, the OH•••O Hbonds in the β-O-4 linkage are broken by the binding of the metal cation, which induces strong electrostatic interactions with the lone pairs on the oxygen atoms. Nevertheless, the syn/ anti R1 rotamers are expected to display similar splittings in their respective origin transitions, and this differentiation is provisionally made at this point between conformer A and the remaining conformers present in each complex. Indeed the lowest four calculated structures can be categorized into two families: the global minimum (1)/anti family and the (1)/syn family at approximately +2.5 kJ/mol. The S0-S1 region of the UVP spectrum (shown in Figure 3a) is assigned to the R1 UV absorption based on its close proximity to the R1 absorption of the neutral.31 The alternative, in which the Li+/Na+ binds between the methoxy groups of ring R2, as it would 1,2-dimethoxybenzene•Li+/Na+, should produce absorption near 37000 cm−1, well outside the region shown.41 All three of the assigned β-O-4•Li+ conformers in Figure 3 have vibronic bands appearing ∼21, 33, 44, and 65 cm−1 above each origin transition (±1−5 cm−1 between conformers). Calculations identify these low frequency vibrations with large amplitude ring torsional (+21 and +33 cm−1) and methoxy torsional modes, the latter of which modulates the position of the Li+ in the binding pocket. By replacing Li+ with the larger sodium cation, the frequencies of these modes are lowered by about 5 cm−1, in keeping with observed experimental shifts. The overview UV spectrum of β-O-4•Li+ shown in Figure 2a displays two other intense sets of transitions nominally +800 and +1400 cm−1 from the origin region, each displaying a congested set of transitions arising from low-frequency vibronic activity. These large absorptions are attributed to Franck− Condon active ring modes, the first of which are likely due to the 6b10 ring deformation and 110 ring-breathing fundamentals common to phenyl derivatives (labeled here using Varsanyi notation for benzene’s modes),42 both of which show strong Franck−Condon factors in the R1 analog 4-methylguaiacol and neutral β-O-4.27,31 The low-frequency vibronic activity built off each ring fundamental is due to combinations with the lowfrequency torsional modes whose fundamentals have already been assigned in the low frequency region. In the β-O-4•Li+ complex, it is possible that the set of transitions centered at 36500 cm−1 (Figure 2a) is at least partly associated with the R2 S0-S2 electronic transition, but no clear assignment is possible based on the Franck−Condon activity.41 On the other hand, when comparing these features to the UVP spectrum of the sodiated complex (Figure 2b), the third set of transitions begin with a prominent, isolated band at 36250 cm−1 (+1193 cm−1 from B 000). Interestingly, the Franck− Condon activity built off of this band differs significantly from the β-O-4•Li+ spectrum, as does the low-frequency activity when compared to the S0-S1 region. As a result, we assign this band in β-O-4•Na+ to the S0-S2 origin, based on its unique vibronic profile. The marked red-shift in the S0-S2 electronic transition compared with the lithium complex is consistent with 1,2-dimethoxybenzene cation complexes,41 a result confirmed by computational predictions, as discussed in the following section. The photofragment mass spectrum (PFMS) of β-O-4•Li+ (m/z 327), shown on the top of Figure 3b, was taken with the UV laser fixed on the S0-S1 origin transition of conformer C of the ion. This mass spectrum is representative of analogous

spectra recorded at each of the UV transitions labeled with asterisks in Figure 2a, which were identical to one another. Analysis of the PFMS of β-O-4•Li+ indicates a primary photofragment product ion at m/z 278 and two less abundant fragments at m/z 294 and 312, respectively. Note that all these channels constitute loss of small stable molecule(s) from the parent ion, without full breakage of the linkage to release one or the other aromatic ring. The high mass peak at m/z 312 is assigned to the loss of methyl radical (−15 Da) from the complex, while the primary photofragment at m/z 278 is assigned to the structure displayed in the figure, a product of elimination of H2O and CH2O (formaldehyde) from the linkage. The difference of one mass unit between the structure in Figure 3b (top) and the PFMS is assumed to be due to formation of the radical ion product caused by loss of the phenolic hydrogen. However, given the mass resolution of the MSAE scan, it is also possible that a slight offset in the mass calibration of the instrument accounts for the discrepancy. These pathways will be discussed in further detail in the Discussion section. By contrast, the PFMS of β-O-4•Na+ (Figure 3b, bottom) shows a drastically different photofragmentation pattern that is formed following excitation of all UV transitions marked by an asterisk in Figure 2b. Photofragment ions now appear at significantly lower mass that must necessarily involve breaking the linkage itself. This result is initially surprising, given the close similarity in the UV spectra that suggest a similar conformational makeup for the Li+ and Na+ complex ions. The fact that such dramatic differences in photofragment pathways accompanies substitution of Na+ for Li+ in the complex points to the presence of different excited state-mediated fragmentation pathways and provides evidence that the alkali metal ion is not simply a bystander in these processes. The mass channel corresponding to the major fragment found in the β-O-4•Li+ PFMS associated with loss of water and formaldehyde, is only barely discernible at m/z 295 in this case. The structure for the low mass fragment at m/z 143 is shown in the figure, and the identity of the major fragment at m/z 173 is tentatively assigned to sodium-bound p-coumaryl alcohol. In the Discussion section, we compare these results with CID studies. ii. IRFIG Spectroscopy. To assess the conformational distribution in the cold ion trap, IRFIGS (IR-UV gain mode) was carried out in the OH stretch region of both β-O-4•metal ion complexes. These IR spectra, which contain contributions from all conformers present in the ion trap, are shown in Figure 4, where they are compared with simulated spectra of the lowest energy structures calculated by DFT. Three OH stretch fundamentals are expected for each conformer contributing to the spectrum, due to the R1 guaiacyl OH•••OCH3 and the two α/γ−OH groups on the β-O-4 linkage (Figure 1). Surprisingly, the IRFIG spectra in Figure 4 exhibit only three bands total, demonstrating that the different conformers present in the cold trap have OH stretch transitions that are nearly identical with one another, consistent with the conformers sharing a single linkage conformation that is essentially unchanged between them. The most intense, broadened band at ∼3587 cm−1 is assigned to the R1 guaiacyl ring OH stretch, which is involved in an OH•••OCH3 Hbond. It should be noted that the frequency of this band compared with G-type neutral molecules is shifted to lower frequency by ∼10 cm−1, likely induced by a weak long-range interaction with the metal ion.30,31,43 1921


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The type-1 binding conformation for the Li+ and Na+ ions uses the oxygen atoms in the β-O-4 linkage as anchors for binding the cation by orienting the oxygen lone pairs toward the (+) charge. This strong interaction prevents the formation of any intramolecular H-bonds in the linkage but also produces the slight shift to lower frequency induced by the interaction of the oxygen atom lone pairs with the larger Na+. Comparison of the experimental spectra with the predictions of calculations for the four low-energy type-1 conformers (Figure S3) confirms the insensitivity of the OH stretch transitions to the slight conformational differences between these conformers. The solvation of the cation is the dominant interaction driving the conformational preferences, aligning the two OH groups with their hydrogens pointing away from the cation, resulting in free OH stretch transitions. The “(a)/(b)” designations following the structure type in Figure S3 are associated with different approach angles of the R2 ether oxygen toward Li+/Na+, manifested in the CαCβOβCφR2 dihedral angle. In all these low-energy conformers the ether and methoxy oxygens on ring R2 play a secondary role in further stabilizing the cation binding site. The simulated spectra reproduce the experimental IRFIG spectrum quite well, as demonstrated in Figure 4. Only slight splittings are calculated for the different conformers, and the shoulder to the blue of the 3655 cm−1 transition is likely evidence of this. IRFIDS was attempted on both samples, but no depletion was observed due to the dominant IRFIG gain signal. The IRFIG spectra verify the extended trans cation-binding conformation of the β-O-4 linkage (type 1) as the dominant conformation observed. However, if the conformational assignment in the UVP spectrum is correct, the UV spectrum is more sensitive than the IR to the more subtle differences between the three (two) conformers of β-O-4•Li+ (β-O4•Na+). In order to test whether the shift in electronic origins can be used as an aid in conformational assignment, TDDFT vertical excitation energy calculations were compared with the experimental origin transitions in Figures 2 and 3a, with the results compiled in Table 1. Such a comparison of experiment origin positions with TDDFT predictions of the relative splittings of the conformers has aided previous analysis of the neutral β-O-4 molecule and related analogs.30,31 Indeed, the

Figure 4. IRFIG spectra of (a) β-O-4•Li+ and (b) β-O-4•Na+ along with simulated spectra of the global minimum structures. IRFIG spectra were collected using a UV wavelength 1−2 nm red of the conformer A origin transitions.

The other two bands at higher frequency are therefore OH stretch transitions of the linkage. These appear at 3655 and 3683 cm−1 for β-O-4•Li+ conformers (Figure 4a), close to the frequency positions of the free α−OH and free γ−OH fundamentals in the β-O-4 analog 1-(4-hydroxy-3methoxyphenyl)propane-1,2,3-triol (HMPPT).30 On this basis, these bands are ascribed to free α−OH and free γ−OH stretch bands that are uniquely present in the type-1 binding conformation. Further verification of this assignment is given in the Supporting Information, where the simulated IR spectra of the three possible binding conformations are compared with experiment (Figure S1). The IRFIG spectrum of β-O-4•Na+ in Figure 4b shows the identical pattern, with free α−OH and free γ−OH bands shifted down in frequency ∼10 cm−1 from their wavenumber positions in β-O-4•Li+.

Table 1. Calculated Energies, Structural Parameters, Experimental S0−S1 Origin Frequencies, and Calculated S0−S1 Origin Frequencies (Scaled to Conformer A β-O-4•Li+) for the Conformers of β-O-4•Li+ and β-O-4•Na+a β-O-4•Li+ electronic origin (cm−1) conformer

E (kJ/mol)

α−OH/OCH3

C B A

0 0.78 2.62 2.89

anti anti syn syn

binding site 1 1 1 1 β-O-4•Na+

CαCβOβCφ

expt

calcd

179° (a) 56.2° (b) 56.3° (b) 179° (a)

35161 (176) 35083 (98) 34985 (0)

35187 (205) 35121 (139) 34982 (0) 35076 (94)

electronic origin (cm−1) conformer

E (kJ/mol)

α−OH/OCH3

binding site

CαCβOβCφ

expt

calcd

B

0 0.11 2.47 2.62

anti anti syn syn

1 1 1 1

64.1° (b) 167° (a) 64.5° (b) 166° (a)

35057 (50)

35073 (58) 35164 (149) 35015 (0)

A

a

35007 (0)

Origin splittings from conformer A are given in parentheses. 1922


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Figure 5. Structures of the assigned conformations of β-O-4•Li+. Dashed red lines represent interactions with the lithium cation.

calculations reproduce the splittings between the conformations’ electronic origins with striking accuracy. Furthermore, the relative intensities of the origin transitions in the UVP spectrum are consistent with the energy ordering of the conformers predicted by the calculations (Table 1). This comparison lends confirming evidence to the assignment of transitions A−C in the UVP spectrum of β-O-4•Li+ (Figure 3a) to conformers A−C, with structures shown in Figure 5. C. Pinoresinol•Li+/Na+ Spectroscopy. i. UVP Spectroscopy and Photofragment Mass Spectrometry. The UV spectrum of neutral, gas-phase pinoresinol displayed a peculiar vibronic profile due to vibronic coupling effects of the two nearly degenerate excited states.31 The UVP spectra of pinoresinol•Li+ and pinoresinol•Na+ are shown in Figure 6a (top and bottom respectively). Immediately evident is the marked difference from β-O-4•Li+ where most of the vibronic transitions were well-resolved and assignable. In the case of pinoresinol•Li+, two absorptions with some discernible structure are found in the low frequency region, with the first origin transition arising at 34959 cm−1 and a second large band at 35199 cm−1 which seems to be built off of a smaller origin unresolved in the spectrum. The first observable origin transition is shifted down in wavenumber by ∼350 cm−1 from the neutral analog, again signaling the perturbative effect of the metal cation. Some low frequency structure is resolved for the small bands near 35000 cm−1, but much of the spectrum is unresolvable, particularly above 35600 cm−1 where the absorption is completely structureless. The spectrum was taken out to ∼36800 cm−1 and is shown in full in the Supporting Information (Figure S4). The high frequency region differs from the neutral, however, which showed a sharp cutoff in the resonant two-photon ionization spectrum within 200 cm−1 of the electronic origin. In the ion, the photofragment signal extends for at least 2000 cm−1, and the two broad features near 36000 and 36500 cm−1 are tentatively assigned to Franck− Condon activity involving ring modes, as in β-O-4•Li+. The two sets of transitions at low frequency are assigned to separate conformers, as will be analyzed further in the following sections. The nESI mass spectrum of the pinoresinol•Li+ solution (shown in the Supporting Information), without precursor ion isolation, showed a second mass peak above the precursor ion

Figure 6. (a) UVP spectra of pinoresinol•Li + (top) and pinoresinol•Na+ (bottom) in the low-frequency Franck−Condon region. (b) PFM spectra of pinoresinol•Li+ (top, m/z 365) and pinoresinol•Na+ (bottom, m/z 382) taken from excitation at transitions labeled in (a).

1923


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The Journal of Physical Chemistry A with nearly equal abundance at m/z ≈ 382, approximately the mass of the monohydrated complex pinoresinol•Li+•H2O (m/ z = 383). By isolating this peak in the nESI spectrum and recording UVP and IRFIG spectra, it was proven to correspond to the monohydrate. Interestingly, in the UVP spectrum (shown in the Supporting Information), both of the low frequency absorptions display sharper, less extended vibronic features reflecting an influence on the electronic transition associated with attachment of a water molecule (presumably to the positive charge). This change imparted to the UVP spectrum by water complexation allows for the recognition of two truly separate sets of transitions, attributed to either two conformers, or separate S0-S1 and S0-S2 electronic transitions from a single conformer. This will be further discussed in the following section. The UVP spectrum of pinoresinol•Na+ is shown in the bottom of Figure 6a. It should be noted at the outset that the nESI signal of the parent complex was diminished by seemingly unavoidable formation of the 1:1 complexes of pinoresinol•Na+ with water and methanol separately. These solvent-containing complexes were not investigated further. The UVP spectrum shows several sharp transitions, beginning with the large band at 35258 cm−1, appearing on top of a large background absorption. Particularly surprising, the absorption stretches nearly 300 cm−1 to the red of the first observable sharp band, indicating that this broad photofragment signal is a real consequence of differences in the UV spectra of different conformational isomers or of the presence of unique excited state processes that affect one of the excited states. These possibilities will be revisited shortly. Nevertheless, bands at +16 and +51 cm−1 above the supposed origin transition are clearly resolved, the first being assigned to a low frequency ring flapping mode. Figure 6b presents photofragment mass spectra of the Li+ and Na + pinoresinol complexes, once again revealing remarkable differences in fragmentation patterns between the two. The PFMS of pinoresinol•Li+ shown in the top portion of Figure 6b shows two primary fragment channels to yield products at m/z 305 and m/z 214. The assigned fragment ion structures are shown in the figure. The m/z 305 fragment is assigned to the structure corresponding to loss of formaldehyde (H2CO) from both methoxy groups on the aromatic rings, with the remaining H atoms attaching to the aromatic rings. The fragments at m/z 214 and m/z 159 are confidently assigned as two separate members of a single fragmentation pathway, with Li+ binding to one or the other fragment. The larger abundance of the m/z 214 product likely reflects a higher binding energy of Li+ in this fragment. Despite the limited information extracted from the UVP spectrum of pinoresinol•Na+, the photofragment mass spectrum shown at the bottom of Figure 6b is quite intriguing. First, the dominant fragment ions at m/z 147 and m/z 359 have no analog in the Li+ spectrum, while the sodiated versions of the major fragments seen in the pinoresinol•Li+ spectrum are just barely discernible. The most abundant fragment peak at m/ z 147 is assigned to the guaiacol•Na+ complex, while the channel at m/z 359 is due to the pinoresinol molecular cation following loss of neutral Na atom. The large abundance of the molecular ion is striking and unprecedented in the series presented here. Its appearance must be the result of electron transfer from the excited state of pinoresinol to the Na+ cation, leading to dissociation of neutral Na from the complex. The involvement of such a photoinduced

electron transfer provides a source for the extensive broadening of the UVP spectrum, as lifetime broadening could emerge as a consequence of the fast rate of this process. This is discussed in greater detail in section IV.C. The appearance of both sharp transitions and the large, broad signal in the UVP spectrum could signal the presence of two conformers that differ in their rates for the electron transfer. It should be noted that in the case of pinoresinol•Li+, sensitivity to the molecular ion is poor due to the limited mass resolution in the experiment (fwhm ∼3 Da) and the small m/z difference between M+ and M•Li+. However, there were no secondary indications of this pathway in the UV spectrum or photofragment mass spectrum. The IRFIG mass spectrum, shown in the Supporting Information, yielded a significant increase in the M+ abundance (relative to m/z = 147), suggesting an increase in the electron-transfer mediated dissociation rate simply by projecting higher onto the S1 surface following the increased internal energy (∼3500 cm−1) associated with absorption of an infrared photon. ii. IR Spectroscopy. Figure 7a compares the conformationspecific IRFID spectrum (IR-UV depletion mode) taken with UV laser fixed on the band at 35199 cm−1, with the IRFIG spectrum of pinoresinol•Li+. The depletion spectrum shows a single OH fundamental at 3585 cm−1, associated with the neardegenerate pair of guaiacyl OH stretch transitions that are unresolved. The IRFIG spectrum below it mirrors this single broad transition, but also contains two other weak bands at 3649 and 3725 cm−1 (labeled with asterisks in the figure), which are too high in frequency to be from pinoresinol•Li+ but suggested the presence of a water molecule. Indeed, close inspection of the parent-ion isolation window used to record the IRFIG spectrum in Figure 7a indicated that a small amount of the pinoresinol•Li+•H2O complex ion (m/z 382) was passed into the cold trap in addition to the main pinoresinol•Li+ complex. An IRFIG spectrum recorded when this peak was intentionally isolated produced the infrared spectrum shown in Figure 7b, ascribable to the pinoresinol•Li+•H2O complex. The transitions at 3649 and 3725 cm−1 are the symmetric and antisymmetric stretch fundamentals of this water-containing complex and reflects a water molecule in which neither OH group is involved in a H-bond. The IRFID spectrum did not suffer from this interference due to its IR-UV double resonance character. To assess the possible conformations of pinoresinol•Li+ present in the 22-pole trap, DFT calculated structures were examined both for their energetic stability and their simulated IR spectra. Only three conformations were found to have energies within 8 kJ/mol of the global minimum, corresponding to perp/perp(syn) (0.00 kJ/mol), perp/perp(anti) (+3.61 kJ/ mol), and perp/anti(anti) (+4.69 kJ/mol) structures, all with the lithium cation bound to the ether oxygens of the β−β linkage. Indeed, the alternative with Li+ bound to the guaiacyl oxygens, would break the OH•••OCH3 H-bond, inconsistent with the observed experimental spectrum. Thus, the β−β linkage is the clearly preferred binding site for Li+. Of the three low-lying conformers, the perp/perp(anti) structure displays the largest predicted splitting between OH bands at only ∼3 cm−1, although the full width at halfmaximum (fwhm) of the experimental feature is ∼8 cm−1, precluding experimental observation. Therefore, if multiple conformers are present, they yield overlapping OH stretch transitions which cannot be distinguished. The simulated spectrum of the global minimum is plotted below the IRFID spectrum in Figure 7a, and the spectrum of the calculated water 1924


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Figure 8. (a) Global minimum structure of pinoresinol•Li+, (b) the 1:1 water complex of pinoresinol•Li+, and (c) the assigned conformer of pinoresinol•Na+.

Table 2. Relative Energies, Structural Parameters, and Calculated S0-S1 and S0-S2 Transition Frequencies with Relative Splittings in Parentheses for Pinoresinol•Li+ Structures

Figure 7. (a) IRFID spectrum recorded with UV laser fixed on the peak of pinoresinol•Li+ at 35199 cm−1 (red) and IRFIG spectrum (black) of pinoresinol•Li+. (b) IRFIG spectrum of pinoresinol•Li+•H2O, and (c) IRFIG spectrum of pinoresinol•Na+. Stick spectra plotted below each spectrum were calculated at the DFT//M05-2X/631+G(d) level of theory.

energy (kJ/mol)a

OCH3/Oα

OCH3/ OCH3

calc.a,b S0-S1

calc.a,b S0S2

0 3.61 4.69

perp/perp perp/perp perp/anti

syn anti anti

35114 (155) 35071 (111) 34959 (0)

35268 35277 35212

a

Calculated at the DFT//M05-2X/6-31+G(d) level of theory. bScaled by 0.8464.

complex built off of that structure is shown at the bottom of Figure 7b. The structures corresponding to these calculated spectra are given in Figure 8. Given the insensitivity of the IR spectroscopy to the distinctive conformational features between the low energy conformers, TDDFT vertical excitation energies were calculated for the three lowest energy conformers and are given in Table 2. Further analysis of the pinoresinol•Li+ UV spectroscopy and assignments is given in the Supporting Information. Based on the evidence given there, we assign the large set of transitions in Figure 6a to the global minimum perp/perp(syn) conformer shown in Figure 8a. The pinoresinol•Li+•H2O complex shown in Figure 8b uses this same global minimum structure. Finally, Figure 7c shows the IRFIG spectrum of the pinoresinol•Na+ complex in the OH stretch region. IRFID

spectroscopy of pinoresinol•Na+ was attempted using multiple resonant UV wavelengths, but the depletion signal was completely obscured even at resonant UV wavelengths by the gain signal produced by the IR excited ions. Therefore, only the IRFIG spectrum was collected in the OH stretch region. The stick spectra below the experimental spectrum are those predicted for the three lowest energy conformers of pinoresinol•Na+ calculated at the DFT//M05-2X/6-31+G(d) level of theory. The IRFIG spectrum of the sodiated ion complex differs from the pinoresinol•Li+ in that two distinct bands are resolved. These bands appear shifted to either side (3583 and 3594 cm−1) of the expected guaiacyl H-bonded OH 1925


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Figure 9. Zero-point corrected relative energies of the structures calculated for β-O-4•Li+, β-O-4•Na+, pinoresinol•Li+, and pinoresinol•Na+ within 25 kJ/mol of the global minima, calculated at the DFT//M05-2X/6-31+G(d) level of theory. Bold lines represent the tentative assignments made in the experiment and colors in β-O-4 columns represent assigned conformers matching figures in the Results section. See text for label designations.

stretch, which appeared at 3587 cm−1 in β-O-4•Li+ and at 3585 cm−1 in pinoresinol•Li+. Given the lack of conformation specificity in the IRFIG spectrum, this pair of transitions could be due to a single conformer with unique transitions for the two OH•••OCH3 groups, or from different conformational isomers. The stick spectra in Figure 7c show small shifts with conformation and OH•••OCH3 group, with the splittings observed for the anti/perp(anti) and perp/perp(anti) structures similar to those observed experimentally. We therefore assign the observed splitting to the presence of these two structures, with the former shown in Figure 8c. As in the calculated structures of the lithium complex, the structures for pinoresinol•Na+ below 5 kJ/mol all incorporate the symmetric binding of the Na+ ion to the center of the β−β linkage.

especially well-suited to selectively photoexcite at the ionbound site. Second, the β-O-4•Li+/Na+ complexes produced UV spectra containing sharp but dense vibronic structure that extend at least 1500 cm−1 above the electronic origin, arising from strong Franck−Condon activity in ring modes and their combinations with active low-frequency modes. The origin transitions of the conformers present in the ion trap were the most intense transitions observed, but extended progressions in low frequency ring and methoxy torsional modes were nearly as strong, leading to a complex UV absorption spectrum (Figure 2). Similar strong vibronic activity was found in neutral β-O-4 dilignol.31 The UV spectra of both pinoresinol complexes was strikingly different from that in β-O-4•Li+/Na+, in that sharp transitions are only just barely discernible on the red edge (pinoresinol•Li+) or imbedded on top of a broad absorption (pinoresinol•Na+), even under the cold conditions of our experiment (Tions ≈ 10 K). In the case of pinoresinol•Li+ the loss of structure is thought to be due to spectral congestion which is present already in the neutral case.31 The partially resolved low frequency portion of the spectrum demonstrates this. The pinoresinol•Na+ complex, however, has a set of sharp transitions that begin at 35258 cm−1, but these sharp bands appear on top of a broad component that extends gradually to the red with no sign of sharp structure. The presence of this inherently broad component signals the presence of a fast nonradiative process which we have tentatively ascribed to electron transfer, a process to which we return in section IV.C. For now, we merely point out that this broadening would complicate the interpretation of UV spectra of larger complexes, possibly masking the spectral markers of other subunits. In this sense, the Li+ complexes of larger oligolignols appear more promising for selective UV excitation along the oligolignol chain. Finally, only in β-O-4•Na+ was it possible to clearly observe the S0-S2 origin due to excitation of ring R2 (Figure 1), which was shifted up about 1200 cm−1 above the S0-S1 origin. This shift to the blue is likely to be reduced when ring R2 is part of a

IV. DISCUSSION A. Spectroscopic Signatures of Ion-Bound Linkages. The UV spectra of the β-O-4 and β−β dilignols bound to a lithium or sodium cation provide an essential foundation for future studies of UV photofragmentation of larger oligolignols where the metal ion can bind at one of several linkages along the oligomer backbone. By comparing the spectral signatures of the ions with their neutral counterparts and of Li+ versus Na+, we explore the potential for selective excitation of individual UV chromophores in a larger oligolignol ion. The results presented here suggest the feasibility of this idea, demonstrating the unique UV absorptions of the different dilignols, and the characteristic ion fragmentation pathways that follow UV excitation. As a first level of distinction, one would like to use the wavelengths of the S0-S1 origins as a means to selectively photoexcite a particular linkage, either in the presence or absence of the alkali metal ion bound to it. Comparison of the spectra of the ion complexes (Figures 2 and 6) with those of the neutral counterparts31 indicates that the metal ion red-shifts the origin transitions by several hundred wavenumbers. This shift to the red associated with ion complexation suggests that the ion-bound site in a larger oligolignol will be readily distinguished from the neutral linkages, with the red-shift 1926


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The Journal of Physical Chemistry A longer oligolignol chain, since the R2 ring will then have further substitution para to the O-4 oxygen atom. In both pinoresinol complexes, the S1−S2 splitting is likely less than 100 cm−1 given the degenerate (or nearly degenerate) electronic states of the equivalent chromophores and comparison with calculated excitonic splittings for neutral pinoresinol.31 This scenario likely contributes to the excessive congestion in the pinoresinol spectra. The IR spectra shown in Figures 4 and 7 provide a straightforward method for distinguishing between the β-O-4 and β−β linkages simply from the number of OH stretch fundamentals, with the former linkage contributing two additional bands. The fact that these two OH stretch fundamentals were free provided strong evidence that the Li+/Na+ ions were bound to the β-O-4 linkage, with both the OH groups oriented so that their oxygen lone pairs solvate the positive charge, breaking any H-bonds that might otherwise be formed. As a result, in larger oligolignols the cation-bound linkage(s) will have shifted OH stretch transitions relative to their neutral β-O-4 counterparts, raising the prospect that the position of the alkali metal ion in the oligolignol could be diagnosed in a straightforward way from the infrared spectrum. B. Alkali Metal Binding Effects and Conformational Landscapes. The conformational assignments made for the observed conformers of the individual dilignol•Li +/Na + complexes involved modest differences in geometry largely associated with reorientations of the two aromatic rings with respect to one another. By contrast, the structural perturbations associated with binding Li+/Na+ to the neutral dilignols involves a whole-sale restructuring of the neutral molecule’s conformational potential energy landscape, reforming it into deep basins corresponding to the different ion binding sites. As has already been noted, the β-O-4 dilignol has two different binding sites to the β-O-4 linkage and a third at the OH/OCH3 groups on ring R1. Within each of these principal binding motifs, several minima exist that differ primarily in the aromatic ring orientations, much as they do in the neutral dilignols themselves. Figure 9 presents energy level diagrams for each of the complexes that display the relative energies of all the conformational minima within 25 kJ/mol of the global minimum. These are based on DFT//M05-2X/6-31+G(d) optimizations, and include zero-point energy corrections. The bold lines indicate the conformational minima assigned based on the IR and UV spectroscopy, with color coding reflecting the colors used in Figures 2, 3, and 5 for β-O-4 complexes. The minima can be separated into two main categories: the lowest energy structures incorporating the guaiacyl OH•••OCH3 Hbond, and the levels starting at ∼20 kJ/mol where this intramolecular H-bond is broken. All the low-lying minima of the β-O-4•Li+/Na+ complexes share the same primary binding motif (labeled “type 1”), in which the Li+/Na+ binds to a three-dimensional pocket involving four oxygen atoms, two on the β-O-4 linkage and the two oxygen atoms on ring R2 (Figure 5). Table 3 presents the calculated binding energies of the global minimum structures for the four complexes. The binding energies were calculated by subtraction of the zero-point corrected energy of the bare molecule and metal ion from the zero-point corrected energy of the corresponding dilignol•cation complex. It is clear from this table that the Li+ cation, with its small size and shorter oxygen-cation binding distance, binds to the dilignols some 0.8−1.0 eV more strongly than their sodiated counterparts.

Table 3. Binding Energies (eV) and Mean Oxygen−Metal Ion Distance (Å) Calculated at the DFT//M05-2X/631+G(d) Level of Theory for the Global Minimum Structures of the Dilignol Complexes binding E. (eV) RO‑M (Å)

β-O-4•Li+

β-O-4•Na+

pino.•Li+

pino.•Na+

3.69 1.97

2.68 2.30

2.66 1.89

1.88 2.26

For the β-O-4 complexes, a second ion binding motif (type-2 binding, shown in the Supporting Information) emerges at +19 and +7.4 kJ/mol respectively, labeled as (2)/syn. The decreased stability of type-2 binding arises from the loss of an Oα•Li+/Na+ interaction in what is now a near-planar threecoordinate binding pocket. In this conformation, the metal ion is positioned between the two R2 oxygens and in the R2 plane, and the γ−OH is configured to interact nearly in the same plane. Finally, this OγOR2Li+(Na+)OR2′ plane is nearly parallel with ring R1, and its π-cloud contributes to the binding energy of the complex. The type-2 β-O-4•Na+ structures are brought down within 7 kJ/mol of the type-1 global minimum, because the Na+ is less solvated in the type-1 binding pocket than is the smaller Li+ (Table 3). Indeed, the calculated binding energies (not shown) for the lowest-energy type-2 conformations are 19 and 7 kJ/mol (0.20 and 0.08 eV) lower than type-1 for β-O4•Li+ and β-O-4•Na+, respectively, consistent with the calculated energies shown in Figure 9. In the pinoresinol•Li+/Na+ complexes, the alkali metal cation also prefers binding to the β−β linkage rather than to the OH/OCH3 secondary sites on the aromatic rings. The two oxygen atoms in the β−β linkage together form a chelating structure for the Li+/Na+, but with total binding energy almost 1.0 eV less than in the β-O-4 linkage. On this basis, we would anticipate that in larger oligolignols, alkali metal cations will prefer to bind to the β-O-4 linkages. The only other ion binding site in pinoresinol is between the oxygen atoms of the guaiacyl rings, similar to dimethoxybenzene-alkali metal complexes.41 While this secondary binding site also provides lone pairs from two oxygen atoms to stabilize the cation binding, the ∼20 kJ/mol H-bond must be broken to effectively complex the ion. As a result, these conformations are higher in energy by 26.8 and 21.2 kJ/mol for the Li+ and Na+ complexes, respectively. Perhaps the more striking aspect of the conformational preferences of the cation complexes are their stark contrast with the neutral dilignols.31 Figure 10 presents the structural changes induced in the preferred neutral conformations by Li+/Na+ binding to form the ion complexes. In the neutrals, the β-O-4 linkage was found to be much more flexible than pinoresinol, with its rigid, bi-ring structure. Indeed, while there were 45 conformational minima of β-O-4 neutral within 20 kJ/mol of the global minimum, only three were found in the same energy range in pinoresinol.31 These three minima in neutral pinoresinol all shared the same β−β linkage conformation (Figure 10b), differing only in the aromatic ring orientations (syn or anti ring/Oα configuration). The β-O-4 linkage, on the other hand, could reconfigure into several nearly isoenergetic conformations with varying H-bonding architectures, ring torsional minima, and linkage backbone structures. Binding to the Li+ or Na+ cations produces a large structural rearrangement of the β-O-4 linkage, because the OH•••O Hbonds along the β-O-4 backbone are broken in response to the strong interactions between the positive charge and the lone 1927


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Positive-mode collision-induced dissociation (CID) data for lithiated or sodiated lignin complexes have been little reported, presumably due to the facility with which negative-mode MSn of lignin compounds can be performed.2,7,19 In order to facilitate a direct comparison of CID and UV excited mass spectra on identical samples, positive-mode CID mass spectra for both β-O-4 complexes were recorded as a part of the present work and are shown in Figure 11. All CID experiments

Figure 10. Structural perturbation induced by complexation with the lithium cation in (a) β-O-4 and (b) pinoresinol. Dashed blue lines represent H-bonds, while dashed red line represent interactions with the lithium cation.

pairs on those nearby oxygens. The major conformer assigned to neutral β-O-4 is compared with the ion complex in Figure 10a to illustrate this perturbation. The degree of stabilization brought on by complexation in this molecule is manifested in the involvement of four oxygen atoms all donating electrons to the positive charge (represented by red dashed bonds). The calculated binding energy of β-O-4•Li+ is substantial at 3.69 eV, which is comparable to crown ether-metal cation complexes (just below 5 eV for the Li+ complex)44 and on the same order as carbon−carbon bond dissociation energies. Pinoresinol on the other hand, prefers an extended bridge structure as neutral (Figure 10b, left), but reshapes this bridge by puckering it so that the two oxygen atoms in the bridge point directly toward the cation charge. This restructuring is somewhat less dramatic than in β-O-4, because no H-bonds need to be broken in reconfiguring the β−β linkage. Interestingly, the polarization of the ether oxygen atoms in the complex does, however, open a third ring torsional minimum in the “perpendicular (perp)” configuration in which the ether oxygen is nearly orthogonal to the ring plane. The perp conformation was not a minimum in the neutral case, and the density of conformational minima in the ion complex substantially increases as a result. Taken as a whole, Li+/Na+ binding imparts a similar degree of structural rigidity to the two linkages, with a similar number of conformational minima within 20 kJ/mol of the global minimum in all four cation-linkage complexes (Figure 9). C. Photofragmentation Pathways. In the Results and Analysis section, specific photofragment product ions were identified for each dilignol ion complex. Here we compare in more detail the UV photofragmentation products with those from collision-induced dissociation (CID) data in order to ascertain the unique aspects of the resonant UV excitation employed in this work. While CID data shows fragmentation from collisionally activated ions with a wide range of energies, UV laser excitation of the cryocooled ions places a precise amount of energy into the ions (35000 cm−1 = 285.7 nm = 4.34 eV = 100 kcal/mol). Nevertheless, if internal conversion to the ground state governs the nonradiative decay, it is expected that product fragments similar to CID will dominate the fragment distribution. Alternatively, if dissociation occurs on excited state surfaces, different mechanisms may arise that contrast with those from CID.

Figure 11. MS2 CID mass spectrum of (a) β-O-4•Li+ and (b) β-O4•Na+ with the structures of the primary fragments given.

were performed on a QTRAP 4000 hybrid triple quadrupole/ linear ion trap mass spectrometer (AB Sciex, Concord, ON, Canada). The lignin ion of interest was isolated in Q3 and subjected to further characterization via ion trap CID and mass analysis using mass-selective axial ejection (MSAE).35 For both β-O-4•Li+ and β-O-4•Na+ complexes, the positivemode CID spectrum under 45 mV activation conditions led to near complete depletion of the precursor ion signal, with little fragmentation yield above m/z = 50. This points to the presence of an efficient CID fragmentation pathway involving loss of the Li+ or Na+ cations, which we are unable to observe in either the CID or UV photofragmentation experiments due to discrimination against low-m/z product ions. The oxygen-rich binding pockets used by the dilignols to bind Li+ or Na+ produce binding energies (1.88−3.69 eV, section IV.B) that are a sizable fraction of a typical chemical bond, but still represent an efficient fragmentation pathway. The primary observed CID fragment from β-O-4•Li+ appears at m/z 279, corresponding to the loss of H2O and CH2O from the β-O-4 linkage. The corresponding product from CID of β-O-4•Na+ yields m/z 295 product ions that dominate its CID spectrum. Interestingly, negative-mode CID of β-O-4 anions also displays the same dominant CH4O2 loss 1928


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The Journal of Physical Chemistry A channel, demonstrating the preference for this neutral loss pathway regardless of the charge.7 UV photofragmentation of β-O-4•Li+ also shows as its primary photofragment the same m/z 279 fragment, pointing to it being formed following internal conversion to form a hot ground state ion which subsequently dissociates. A secondary product shown in the PFMS in the top of Figure 3b is the product following methyl radical loss (m/z 312), likely from the guaiacyl ring (R1). Only a very minor yield of this product was found in CID. While β-O-4•Li+ photoexcitation yielded similar fragment ions to those produced by positive-mode CID, the photoproducts from β-O-4•Na+ (Figure 3b) are completely unique to the UV photoexcitation process. There is almost no sign of the m/z 295 fragment that dominated CID, but a range of intermediate mass primary fragments are formed, the most abundant of which are tentatively assigned to p-coumaryl alcohol•Na+ (m/z 173) and 4-vinylphenol•Na+ (m/z 143). Since these products are unique to UV photoexcitation, an explanation for their formation involves excited state processes that present a challenge to theory to explain. Fortunately, the present data provides a well-defined starting geometry for the fragmentation process, with initial excitation localized on the R1 ring (Figure 1) that is not directly bound to the Li+/Na+ (Figure 5). The pinoresinol complexes yielded photofragmentation mass spectral patterns substantially different than those identified in the β-O-4 counterpart. In pinoresinol•Li+, the primary fragment ions appear at m/z 214 and m/z 305. The m/z 214 and m/z 159 ions are assigned to a single fragmentation pathway involving dissociation of the Cγ-O bond of one side of the linkage and Cα-Cβ on the other side. The detected fragments are from separate events where the lithium ion binds to one or the other fragment. The ion at m/z 214 is one mass unit less than the projected structure in Figure 6b (m/z 215), and it is assumed that this is due to hydrogen loss from the guaiacyl OH. The fragment at m/z 305 from pinoresinol•Li+ (Figure 6b) was assigned to loss of neutral formaldehyde from both of the guaiacyl methoxy groups. For pinoresinol•Na+, the loss of one guaiacol ring via Cφ‑Cα dissociation (to form m/z 147) is a primary pathway following UV excitation, but the peculiar appearance of the molecular cation (m/z 359) is of primary interest. This pathway, shown in pictorial form in Figure 12a, was discussed in section III.C.i as an electron-transfer process resulting in loss of neutral Na atom from the complex. Just such a mechanism was postulated to account for the Na+-specific spectral broadening observed among the alkali metaldimethoxybenzene series studied by Inokuchi et al.41 In that case, it was surmised that fast internal conversion to the Na + M+ surface occurred well above the dissociation asymptote following excitation to the Na+•M S1 state, leading to facile dissociation immediately following electron transfer. We infer an analogous mechanism in the case of pinoresinol•Na+. Despite the similar energetics anticipated in the two cases, the electron transfer pathway operates efficiently in pinoresinol•Na+ but is not detected in β-O-4•Na+. Figure 12b compares the energetics of the electron transfer process in the pinoresinol•Na+ and β-O-4•Na+ cases. The energies shown in Figure 12b were found following the procedure reported by Inokuchi et al.,41 where the known ionization potential (IP) of sodium is 5.14 eV45 and the IPs of the dilignols were calculated using the neutral global minimum

Figure 12. (a) Pictorial representation of photoinitiated electron transfer/dissociation in pinoresinol•Na+. (b) Energy level scheme of dissociation asymptotes and transition energies for pinoresinol•Na+ and β-O-4•Na+ calculated following the procedure in Inokuchi et al.41

conformations and were found to be 7.57 and 7.47 eV for pinoresinol and β-O-4 respectively. As illustrated in Figure 12b, by taking the difference between dilignol and sodium IPs and calculating the ground state binding energy, the electron transfer energy can be inferred relative to the ground state of the complex. Indeed the origin transition of the pinoresinol complex is just above the calculated pinoresinol+ + Na asymptote with a difference of ∼0.06 eV, or 500 cm−1, indicating the availability of this pathway following photoexcitation to the excited state of the complex. This differs from the β-O-4 complex (Figure 12b, right), which has an electron transfer asymptote 5.0 eV above the ground state and is thus endothermic by over 0.6 eV at the S0-S1 origin. This difference between the pinoresinol and β-O-4 complexes with Na+ occurs primarily because of the difference in binding energies of the cation to the linkages. It should be noted that the binding energy of the pinoresinol•Na+ complex is near that reported for 1,2-dimethoxybenzene•Na+ (1.82 eV). The corresponding electron transfer reaction in the Li+ complexes is also endothermic by about 5000−12000 cm−1 relative to the S1 state, in keeping with the fact that the molecular cation produced by loss of Na is energetically accessible only in pinoresinol•Na+ in this series. Finally, it is worth highlighting once again the significant enhancement of the molecular ion product compared to the other major 147 Da fragment when the ion complex is excited to S1 after IR excitation (IRFIGS). 1929


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presented, differing only in the orientation of each guaiacyl ring with respect to internal rotation about the Cφ-Cα bond. The structural perturbation to the neutral dilignols induced by complexation to the Li+/Na+ cation is significant, especially for the β-O-4 dilignol. A computational investigation of all lowlying conformational minima shows that the strong binding of the alkali metal cation to the β-O-4 linkage locks in this conformation, effectively reducing the flexibility of the β-O-4 linkage so that, in the presence of Li+/Na+, it is comparable to the β−β linkage in conformational landscape. Finally, the resonant UV photofragment mass spectra are demonstrated to be completely unique to each linkage/alkali metal combination. Of particular interest was the remarkable difference in fragment ions compared with conventional CID methods, particularly for the two sodium complexes studied here. The detection of the molecular ion following UV excitation of pinoresinol•Na+ supports an electron-transfer mediated dissociation pathway specific to the sodiated complex. The specific photofragmentation pathways provide easy-toidentify markers that raise the prospects for site-selective excitation and site-localized fragmentation in a larger lignin oligomer, motivating future efforts along this direction.

The difference PFMS is shown in Figure S7 and shows nearly exclusively molecular ion formation, suggesting an enhancement in electron transfer by starting higher in energy on the S1 surface. It would be instructive to compare the PFMS results discussed here with “fast” and “slow” CID activation methods of the dilignol•Li+/Na+ complexes, a task left for future work.46 The photofragmentation patterns we have just described provide a second dimension along which sequencing an oligolignol chain could be achieved. Resonant UV excitation of a single chromophore in the oligolignol chain would initiate this fragmentation from different starting points, but the different fragmentation pathways for the β-O-4 and β−β linkages offer exciting possibilities for sequencing. It is noteworthy that the mass spectra in Figures 3b and 6b possess almost no overlap in fragment ions produced by the four combinations of linkage and alkali metal ion. The β-O-4•Li+ complex was characterized primarily by the loss of CH4O2 (presumably as water + formaldehyde) from the β-O-4 linkage following UV excitation, while β-O-4•Na+, pinoresinol•Na+, and pinoresinol•Li+ complexes yielded lower mass fragments involving breaking the β-O-4 or β−β linkages, a necessity for efficient sequencing. What is not known at this point is whether electronic excitation of a particular chromophore in the oligolignol chain will lead to selective fragmentation of that linkage or whether electronic energy transfer along the oligolignol chain will dominate. If fragmentation is localized, then it should be possible to tune through the UV spectrum, with changes in fragmentation pattern reflecting the chromophore responsible for excitation. This is a fascinating prospect awaiting future work.

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ASSOCIATED CONTENT

S Supporting Information *

Comparison between calculated β-O-4 binding families and low-energy conformers, the extended pinoresinol•Li+ UVP spectrum, pinoresinol•Li+•H2O UVP spectroscopy, IRFIG PFMS of pinoresinol•Na +, and detailed discussion of pinoresinol•Li+ UV spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

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V. CONCLUSIONS This work presents the first coupled optical spectroscopy/mass spectrometric study of model dilignol ions. The ions were cooled in a cryo-cooled 22-pole trap to temperatures below 10 K, ensuring that the UV and IR spectra came from low-lying conformational minima in their vibrational zero-point levels. UV photofragmentation spectra, photofragment mass spectra, and IR spectra were acquired for G-type β-O-4 and β−β linkages complexed with lithium or sodium cations. Unique spectral signatures were obtained for each of four combinations of linkage-alkali metal complexes, along with their corresponding photofragmentation patterns. The combined data from the three methods provides a strong foundation for future studies that seek to use UV photofragmentation as a tool for sequencing larger lignin oligomers. Analysis of the UVP spectrum, in conjunction with TDDFT vertical excitation energies, led to the assignment of three conformers of β-O-4•Li+ and at least two conformers of β-O4•Na+. IRFIG spectroscopy was used to determine that the ion binding conformation of all three conformers is in a (nominally) four-coordinate binding pocket in the β-O-4 linkage, with both linkage OH groups and the OH/OCH3 groups on the R2 aromatic ring donating their lone pair electrons to stabilize the positive charge. The flexibility of the βO-4 linkage permits a three-dimensional solvation of the positive charge by the approach of the four oxygen donors along multiple axes. This maximizes the binding energy of the complex, which was calculated to be D0 = 3.69 eV in the lithiated complex. In pinoresinol, the primary binding site is in the center of a puckered form of the β−β ether linkage where its two ether oxygens can interact with the alkali metal cation. Evidence for at least two conformers of pinoresinol•Li+ was

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Addresses

† Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada. ‡ Kalsec, P.O. Box 50511, Kalamazoo, MI 49006 United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge insightful discussions with Hilkka Kenttämaa regarding mass spectrometry of lignin-alkali metal cation complexes, and Bidyut Biswas for synthesis assistance. The authors also acknowledge support for this work from the Department of Energy Basic Energy Sciences, Division of Chemical Sciences (DEFG02-96ER14656 and DEFG0200ER15105).

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REFERENCES

(1) Boerjan, W.; Ralph, J.; Baucher, M. Lignin Biosynthesis. Annu. Rev. Plant Biol. 2003, 54, 519−546. (2) Morreel, K.; Dima, O.; Kim, H.; Lu, F. C.; Niculaes, C.; Vanholme, R.; Dauwe, R.; Goeminne, G.; Inze, D.; Messens, E.; Ralph, J.; Boerjan, W. Mass Spectrometry-Based Sequencing of Lignin Oligomers. Plant Physiol. 2010, 153, 1464−1478. (3) Moura, J. C. M. S.; Bonine, C. A. V.; Viana, J. D. F.; Dornelas, M. C.; Mazzafera, P. Abiotic and Biotic Stresses and Changes in the Lignin Content and Composition in Plants. J. Integr. Plant Biol. 2010, 52, 360−376. 1930


Article

The Journal of Physical Chemistry A (4) Bhuiyan, N. H.; Selvaraj, G.; Wei, Y. D.; King, J. Gene Expression Profiling and Silencing Reveal That Monolignol Biosynthesis Plays a Critical Role in Penetration Defence in Wheat against Powdery Mildew Invasion. J. Exp. Bot. 2009, 60, 509−521. (5) Chabannes, M.; Ruel, K.; Yoshinaga, A.; Chabbert, B.; Jauneau, A.; Joseleau, J. P.; Boudet, A. M. In Situ Analysis of Lignins in Transgenic Tobacco Reveals a Differential Impact of Individual Transformations on the Spatial Patterns of Lignin Deposition at the Cellular and Subcellular Levels. Plant J. 2001, 28, 271−282. (6) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. The Occurrence and Reactivity of Phenoxyl Linkages in Lignin and Low Rank Coal. J. Anal. Appl. Pyrol. 2000, 54, 153−192. (7) Morreel, K.; Kim, H.; Lu, F. C.; Dima, O.; Akiyama, T.; Vanholme, R.; Niculaes, C.; Goeminne, G.; Inze, D.; Messens, E.; Ralph, J.; Boerjan, W. Mass Spectrometry-Based Fragmentation as an Identification Tool in Lignomics. Anal. Chem. 2010, 82, 8095−8105. (8) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The Path Forward for Biofuels and Biomaterials. Science 2006, 311, 484−489. (9) Vanholme, R.; Demedts, B.; Morreel, K.; Ralph, J.; Boerjan, W. Lignin Biosynthesis and Structure. Plant Physiol. 2010, 153, 895−905. (10) Simmons, B. A.; Logue, D.; Ralph, J. Advances in Modifying Lignin for Enhanced Biofuel Production. Curr. Opin. Plant Biol. 2010, 13, 313−320. (11) Weng, J. K.; Li, X.; Bonawitz, N. D.; Chapple, C. Emerging Strategies of Lignin Engineering and Degradation for Cellulosic Biofuel Production. Curr. Opin. Biotechnol. 2008, 19, 166−172. (12) Hilal, M.; Parrado, M. F.; Rosa, M.; Gallardo, M.; Orce, L.; Massa, E. M.; Gonzalez, J. A.; Prado, F. E. Epidermal Lignin Deposition in Quinoa Cotyledons in Response to UV-B Radiation. Photochem. Photobiol. 2004, 79, 205−210. (13) Ralph, J. Hydroxycinnamates in Lignification. Phytochem. Rev. 2010, 9, 65−83. (14) Kishimoto, T.; Uraki, Y.; Ubukata, M. Easy Synthesis of Beta-O4 Type Lignin Related Polymers. Org. Biomol. Chem. 2005, 3, 1067− 1073. (15) Saito, K.; Kato, T.; Takamori, H.; Kishimoto, T.; Fukushima, K. A New Analysis of the Depolymerized Fragments of Lignin Polymer Using TOF-SIMS. Biomacromolecules 2005, 6, 2688−2696. (16) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110, 3552−3599. (17) Stewart, J. J.; Akiyama, T.; Chapple, C.; Ralph, J.; Mansfield, S. D. The Effects on Lignin Structure of Overexpression of Ferulate 5Hydroxylase in Hybrid Poplar. Plant Physiol. 2009, 150, 621−635. (18) Evtuguin, D. V.; Amado, F. M. L. Application of Electrospray Ionization Mass Spectrometry to the Elucidation of the Primary Structure of Lignin. Macromol. Biosci. 2003, 3, 339−343. (19) Haupert, L. J.; Owen, B. C.; Marcum, C. L.; Jarrell, T. M.; Pulliam, C. J.; Amundson, L. M.; Narra, P.; Aqueel, M. S.; Parsell, T. H.; Abu-Omar, M. M.; Kenttamaa, H. I. Characterization of Model Compounds of Processed Lignin and the Lignome by Using Atmospheric Pressure Ionization Tandem Mass Spectrometry. Fuel 2012, 95, 634−641. (20) Owen, B. C.; Haupert, L. J.; Jarrell, T. M.; Marcum, C. L.; Parsell, T. H.; Abu-Omar, M. M.; Bozell, J. J.; Black, S. K.; Kenttamaa, H. I. High-Performance Liquid Chromatography/High-Resolution Multiple Stage Tandem Mass Spectrometry Using Negative-Ion-Mode Hydroxide-Doped Electrospray Ionization for the Characterization of Lignin Degradation Products. Anal. Chem. 2012, 84, 6000−6007. (21) Redwine, J. G.; Davis, Z. A.; Burke, N. L.; Oglesbee, R. A.; McLuckey, S. A.; Zwier, T. S. A Novel Ion Trap Based Tandem Mass Spectrometer for the Spectroscopic Study of Cold Gas Phase Polyatomic Ions. Int. J. Mass Spectrom. 2013, 348, 9−14. (22) Rizzo, T. R.; Stearns, J. A.; Boyarkin, O. V. Spectroscopic Studies of Cold, Gas-Phase Biomolecular Ions. Int. Rev. Phys. Chem. 2009, 28, 481−515.

(23) Stearns, J. A.; Guidi, M.; Boyarkin, O. V.; Rizzo, T. R. Conformation-Specific Infrared and Ultraviolet Spectroscopy of Tyrosine-Based Protonated Dipeptides. J. Chem. Phys. 2007, 127. (24) Stearns, J. A.; Mercier, S.; Seaiby, C.; Guidi, M.; Boyarkin, O. V.; Rizzo, T. R. Conformation-Specific Spectroscopy and Photodissociation of Cold, Protonated Tyrosine and Phenylalanine. J. Am. Chem. Soc. 2007, 129, 11814−11820. (25) Rodrigo, C. P.; James, W. H.; Zwier, T. S. Single-Conformation Ultraviolet and Infrared Spectra of Jet-Cooled Monolignols: PCoumaryl Alcohol, Coniferyl Alcohol, and Sinapyl Alcohol. J. Am. Chem. Soc. 2011, 133, 2632−2641. (26) Vanderhage, E. R. E.; Boon, J. J.; Steenvoorden, R.; Weeding, T. L. Resonance-Enhanced Multiphoton Ionization Mass-Spectrometric Analysis of Lignin Using Laser Pyrolysis with Entrainment into a Supersonic Jet. Anal. Chem. 1994, 66, 543−550. (27) Dean, J. C.; Navotnaya, P.; Parobek, A. P.; Clayton, R. M.; Zwier, T. S. Ultraviolet Spectroscopy of Fundamental Lignin Subunits: Guaiacol, 4-Methylguaiacol, Syringol, and 4-Methylsyringol. J. Chem. Phys. 2013, 139, 144313. (28) Berden, G.; Meerts, W. L.; Schmitt, M.; Kleinermanns, K. High Resolution UV Spectroscopy of Phenol and the Hydrogen Bonded Phenol-Water Cluster. J. Chem. Phys. 1996, 104, 972−982. (29) Song, K.; Hayes, J. M. Supersonic Jet Spectra of ParaAlkylphenols. J. Mol. Spectrosc. 1989, 134, 82−97. (30) Dean, J. C.; Buchanan, E. G.; James, W. H.; Gutberlet, A.; Biswas, B.; Ramachandran, P. V.; Zwier, T. S. Conformation-Specific Spectroscopy and Populations of Diastereomers of a Model Monolignol Derivative: Chiral Effects in a Triol Chain. J. Phys. Chem. A 2011, 115, 8464−8478. (31) Dean, J. C.; Walsh, P. S.; Biswas, B.; Ramachandran, P. V.; Zwier, T. S. Single-Conformation UV and IR Spectroscopy of Model G-Type Lignin Dilignols: The Beta-O-4 and Beta-Beta Linkages. Chem. Sci. 2014, 5, 1940−1955. (32) Roy, S. C.; Rana, K. K.; Guin, C. Short and Stereoselective Total Synthesis of Furano Lignans (±)-Dihydrosesamin, (±)-Lariciresinol Dimethyl Ether, (±)-Acuminatin Methyl Ether, (±)-Sanshodiol Methyl Ether, (±)-Lariciresinol, (±)-Acuminatin, and (±)-Lariciresinol Monomethyl Ether and Furofuran Lignans (±)-Sesamin, (±)-Eudesmin, (±)-Piperitol Methyl Ether, (±)-Pinoresinol, (±)-Piperitol, and (±)-Pinoresinol Monomethyl Ether by Radical Cyclization of Epoxides Using a Transition-Metal Radical Source. J. Org. Chem. 2002, 67, 3242−3248. (33) Redwine, J. G., Purdue University, 2013. (34) Han, H. L.; Londry, F. A.; Erickson, D. E.; McLuckey, S. A. Tailored-Waveform Collisional Activation of Peptide Ion Electron Transfer Survivor Ions in Cation Transmission Mode Ion/Ion Reaction Experiments. Analyst 2009, 134, 681−689. (35) Londry, F. A.; Hager, J. W. Mass Selective Axial Ion Ejection from a Linear Quadrupole Ion Trap. J. Am. Soc. Mass Spectrom. 2003, 14, 1130−1147. (36) Buchanan, E. G.; James, W. H.; Gutberlet, A.; Dean, J. C.; Guo, L.; Gellman, S. H.; Zwier, T. S. Single-Conformation Spectroscopy and Population Analysis of Model Gamma-Peptides: New Tests of Amide Stacking. Faraday Discuss. 2011, 150, 209−226. (37) Weiner, P. K.; Kollman, P. A. Amber - Assisted Model-Building with Energy Refinement - a General Program for Modeling Molecules and Their Interactions. J. Comput. Chem. 1981, 2, 287−303. (38) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. Macromodel - an Integrated Software System for Modeling Organic and Bioorganic Molecules Using Molecular Mechanics. J. Comput. Chem. 1990, 11, 440−467. (39) Zhao, Y.; Truhlar, D. G. Density Functionals for Noncovalent Interaction Energies of Biological Importance. J. Chem. Theory Comput. 2007, 3, 289−300. (40) Frisch, M. J.; et al. Gaussian 09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2009. 1931


Article

The Journal of Physical Chemistry A (41) Inokuchi, Y.; Boyarkin, O. V.; Ebata, T.; Rizzo, T. R. UV and IR Spectroscopy of Cold 1,2-Dimethoxybenzene Complexes with Alkali Metal Ions. Phys. Chem. Chem. Phys. 2012, 14, 4457−4462. (42) Varsanyi, G. Assignments for Vibrational Spectra of 700 Benzene Derivatives; Wiley: New York, 1974. (43) Longarte, A.; Redondo, C.; Fernandez, J. A.; Castano, F. IR/UV and UV/UV Double-Resonance Study of Guaiacol and Eugenol Dimers. J. Chem. Phys. 2005, 122, 164304. (44) Inokuchi, Y.; Boyarkin, O. V.; Kusaka, R.; Haino, T.; Ebata, T.; Rizzo, T. R. Ion Selectivity of Crown Ethers Investigated by UV and IR Spectroscopy in a Cold Ion Trap. J. Phys. Chem. A 2012, 116, 4057−4068. (45) Lias, S. G.; Liebman, J. F. Ion Energetics Data. NIST Chemistry Webbook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD. (46) Wells, J. M.; McLuckey, S. A. Collision-Induced Dissociation (CID) of Peptides and Proteins. Biological Mass Spectrometry; Burlingame, A. L., Ed.; Elsevier Academic Press Inc: San Diego, CA, 2005; Vol. 402, pp 148−185.

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UV Photofragmentation and IR Spectroscopy of Cold, G-Type Î²-O-4

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