UVâVis Action Spectroscopy Reveals a Conformational Collapse in

Letter pubs.acs.org/JPCL

UV−Vis Action Spectroscopy Reveals a Conformational Collapse in Hydrogen-Rich Dinucleotide Cation Radicals Joseph A. Korn,† Jan Urban,‡ Andy Dang,† Huong T. H. Nguyen,† and František Tureček*,† †

Department of Chemistry, University of Washington, Bagley Hall, Seattle, Washington 98195-1700, United States Metagenics, Inc., Gig Harbor, Washington 98335-3729, United States

‡

S Supporting Information *

ABSTRACT: We report the generation of deoxyriboadenosine dinucleotide cation radicals by gas-phase electron transfer to dinucleotide dications and their noncovalent complexes with crown ether ligands. Stable dinucleotide cation radicals of a novel hydrogen-rich type were generated and characterized by tandem mass spectrometry and UV−vis photodissociation (UVPD) action spectroscopy. Electron structure theory analysis indicated that upon electron attachment the dinucleotide dications underwent a conformational collapse followed by intramolecular proton migrations between the nucleobases to give species whose calculated UV−vis absorption spectra matched the UVPD action spectra. Hydrogen-rich cation radicals generated from chimeric riboadenosine 5′-diesters gave UVPD action spectra that pointed to novel zwitterionic structures consisting of aromatic π-electron anion radicals intercalated between stacked positively charged adenine rings. Analogies with DNA ionization are discussed.

I

nteraction of DNA with ionizing radiation,1 reactive oxidative species,2 or metal complexes3 can lead to the formation of cation radical intermediates where the electron defect (hole) is initially located in a nucleobase. The hole is known to migrate along the DNA backbone to intermittently or finally reside on guanine as the nucleobase of the lowest ionization energy.4 Hole formation and propagation in DNA have been studied by various ingenious techniques,5,6 including photoexcitation of intercalated ruthenium complexes,7 oxidation of noncovalent adducts with organic molecules,8,9 and direct ionization in solution.10 Despite the wealth of information that these studies have generated regarding the DNA behavior at the nanoscale level, atomic-level characterization of the process has only been achieved by recent theoretical calculations.11,12 These calculations indicated that proton transfer can be an important component of interactions between a nucleobase cation radical and its nucleobase counterpart in a Watson−Crick double-strand helix.12 In contrast, experimental data probing the intrinsic nature of electron and proton transfer between nucleobases have only recently become available for the guanine-cytosine pair.13 We now report a combined experimental and computational study aimed at achieving atomic-level resolution in characterizing proton and electron transfer along a single strand in DNA and related cation radicals. As the first model systems, we chose deoxyadenosine dinucleotide (dAA, 1) and synthetic chimeric adenine riboside diesters 2−5 containing rigid aromatic and trans-1,4-cyclohexane spacers separating the nucleosides. Double protonation by electrospray ionization places the protons at the most basic positions in the adenine units, which are nearly equivalent but asymmetrically positioned in 1 and symmetrically positioned in 2−5. The dications are © XXXX American Chemical Society

converted to transient cation radicals by electron transfer in the gas phase such that one adenine unit carries an additional hydrogen atom forming a radical and the other unit carries a proton. We call such species hydrogen-rich DNA or chimeric cation radicals by analogy with protein cation radicals generated in a similar manner.14 The DNA-nucleotide (dAA) and RNAchimeric cation radicals are probed by laser photodissociation UV−vis action spectroscopy and tandem mass spectrometry. Action spectroscopy is a photodissociative method that detects resonant absorption of one or multiple photons by a gas-phase ion. Photodissociation (“action”) is monitored by mass spectrometry in a wavelength-dependent fashion, and the obtained action spectra are representative of the electronic excitations in the gas-phase ion.15,16 Several nucleobase radicals have been previously generated by femtosecond electron transfer in the gas phase, but no spectroscopic characterization has been accomplished.17−20 Nucleobase cation radicals have also been produced by intramolecular electron transfer in transition metal complexes and studied by ion−molecule reactions and UV21 or infrared multiphoton action spectroscopy.13,22,23 Here, we introduce a methodology for generating more complex dinucleotide radical systems, allowing us to study intramolecular interactions between the nucleobase cations and radicals of relevance to the broader topic of nucleic acid ionization. Received: July 18, 2017 Accepted: August 15, 2017 Published: August 15, 2017 4100

DOI: 10.1021/acs.jpclett.7b01856 J. Phys. Chem. Lett. 2017, 8, 4100−4107

Letter

The Journal of Physical Chemistry Letters

protonation at the remote N-1 and N-1″ minimized Coulomb repulsion between the charged adenine units (Figures 1 and S1,

Figure 1. ωB97X-D/6-31+G(d,p) optimized structures of dications (1a+2H)2+, (2a+2H)2+, (2b+2H)2+, and (5a+2H)2+. Atom color coding is as follows: turquoise = C, gray = H, blue = N, red = O, pink = P. Only exchangeable (O−H, N−H) hydrogen atoms are shown to avoid clutter. Green arrows indicate major adenine hydrogen bonds. For additional dication structures, see Figure S1 and S2 (Supporting Information).

Nucleotide Protonation. The protonation step in the formation of ion precursors raises the question of protonation sites in the adenine moieties in dications (1+2H)2+ through (5+2H)2+, where each adenine unit is singly protonated. The N-1, N-3, N7, N-9, and NH2 (N-10) positions have been previously considered as potential protonation sites in gaseous adenine,24−26 2′-deoxyadenosine, and adenosine.27 Our density functional theory (DFT) and perturbational Møller−Plesset (MP2) calculations (Table 1) showed the lowest free energy for the N-1, N-1″ protonated dAA tautomer (1a+2H)2+, which is consistent with the energetics of adenine protonation in the gas phase24 but differs from singly protonated adenosine that prefers protonation at N-3 because of stabilization by hydrogen bonding to the oxygen atoms of the sugar moiety.27 All (1+2H)2+ tautomers preferred extended conformations where

Supporting Information). In contrast, the riboadenosine chimeras, (2+2H)2+, showed the lowest energy for the N-3, N-3″ tautomer (2a+2H)2+ (Table S1), which develops hydrogen bonds between the adenine N-3−H, N-3″−H and ribose O-2 hydroxyls (Figures 1 and S2, Supporting Information). The N-3, N-3″ tautomer (5a+2H)2+ was also the lowest-energy dication for (5+2H)2+ ions, where the N-3− H and N-3″−H developed hydrogen bonds to the ester CO (Figure 1). The calculated structures indicate that the different H-bonding in (5a+2H)2+ is due to the rotation of the ester groups out of the cyclohexane ring plane. This is energetically unfavorable in the aromatic diesters (2+2H)2+, (3+2H)2+, and (4+2H)2+ where a near-coplanar conformation of the ester groups and the ring is preferred due to π-electron conjugation. Formation of Cation Radicals. Electron transfer to (1+2H)2+ by a gas-phase ion−ion reaction with a fluoranthene anion radical resulted in extensive dissociation by loss of H (m/z 565), adenine (m/z 431), and C5H7O2 (m/z 332,; Figure 2a). The latter are analogous to the w fragment ions known from dissociations of oligonucleotide even-electron ions.28 To generate abundant (1+2H)+•, we resorted to a technique previously developed for the synthesis of hydrogen-rich peptide cation radicals.29 Noncovalent complexes of (1+2H)2+ with 18crown-6-ether (CE) or its 2,3:11,12-dibenzo analogue (DBCE) were produced by electrospray, isolated by mass (m/z 463 for the DBCE complex) and subjected to electron-transfer reduction with fluoranthene anion radicals. This resulted in efficient elimination of the neutral crown-ether ligand (Figure 2b), forming intact (1+2H)+• at m/z 566 in excellent yield (Figure 2c,d). The same technique was employed to generate (2+2H)+•, (3+2H)+•, and (4+2H)+•, as shown in Figures S3− S5 (Supporting Information). Interestingly, (5+2H)+• was best formed by direct electron transfer to (5+2H)2+, which produced the cation radical with an ion intensity sufficient for action spectroscopy measurements (Figure S6, Supporting Information). We note that the crown-ether complexes of (1+2H)2+ showed energetically favored coordination of CE by hydrogen bonding to the N-1-H or N-1″-H protonated nucleobases (Table S2, Figure S7). The relative energies of

Table 1. Relative Energies of Dications (1+2H)2+ relative energya,b protonation sites

ion 2+

(1a+2H) (1b+2H)2+ (1c+2H)2+ (1d+2H)2+ (1e+2H)2+ (1f+2H]2+ (1g+2H]2+ (1h+2H]2+ (1i+2H]2+ (1j+2H]2+ (1k+2H]2+

1,1″ 1,3″ 3,1″ 3,3″ 1,9″ 9,9″ 1,10″ 10,1″ 10,3″ 3,10″ 10,9″

B3LYP f

0 (0) 4.2 (9.1) 15 (16) 20 (25) 25 (28) 33 (34)

ωB97X-Dc M06-2Xd 0 7.6 14 22 23 36 88 94 103 109 115

0 6.7 15 23 25 40

MP2e 0 11 17 28 26 43

In kJ mol−1. bIncluding B3LYP/6-31+G(d,p) zero-point energy corrections and referring to 0 K. cFrom ωB97X-D/6-311++G(2d,p) single-point energy calculations on ωB97X-D/6-31+G(d,p) optimized geometries. dFrom M06-2X/6-311++G(2d,p) single-point energy calculations on M06-2X/6-31+G(d,p) optimized geometries. eFrom MP2(frozen core)/6-311++G(2d,p) single-point energy calculations on B3LYP/6-31+G(d,p) optimized geometries. fRelative free energies at 298 K are in parentheses. For ion structures, see Figure S1 (Supporting Information). a

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computational analysis of cation radical structures and excitations, as shown in Figure 3b−e and described below. Dinucleotide Cation Radical Structures. Electron-transfer reduction of (1+2H)2+ results in the removal of Coulomb repulsion between the protonated adenine rings, which is replaced by their mutual ion−dipole attraction in (1+2H)+•. Because the adenine moieties are not identical in (1+2H)2+ and electron attachment can occur in either of them, different cation radicals can be initially formed, undergoing further structure development upon geometry relaxation. DFT geometry optimization of the cation radical derived from the most stable dication (1a+2H)2+ led to structure (1a+2H)+•, in which the odd electron was delocalized within the 5′-adenine ring, as illustrated by the calculated spin and charge densities and also indicated by the out-of-plane arrangement of the adenine radical peripheral hydrogen atoms (Figure 4). The structure shows that one-electron reduction forming (1a +2H)+• was accompanied by a collapse of the extended dication geometry by folding at the phosphodiester hinge, resulting in the formation of strong internucleobase hydrogen bonds of N-1″−H to N-9 and N-10″−H to N-10 (Figure 4). A similar collapse occurred upon charge reduction of the other dications (Figure S11, Supporting Information). The strong H-bonding interactions between the protonated and reduced nucleobases, allowed by the conformational collapse in (1+2H)+•, raised the question of proton and hydrogen atom migrations isomerizing the charge-reduced cation radical. If exothermic, such isomerizations could occur spontaneously and be driven by the excitation provided by the exothermic electron attachment to the dication−CE complex.31 For (1a+2H)+• formation by electron transfer to (1a+CE +2H)2+ and the associated loss of CE, we calculated ΔHrxn,0 = −337 kJ mol−1 (eq 1, Supporting Information) which is partitioned among the products. We investigated by DFT calculations several cation radical structures that were derived from the respective (1+2H)2+ tautomers, as summarized in Table S3 (Supporting Information). Interestingly, electron attachment to these dications occurred in the adenine ring protonated in positions other than N-1 (Figure S11, Supporting Information). Among the low-energy cation radicals, (1a +2H)+•, (1b+2H)+•, and (1c+2H)+• (Figure 4) were close in energy and could coexist under equilibrium conditions of proton or hydrogen atom transfers between the adenine nuclei. Note, however, that hydrogen atom migrations within the adenine ring face substantial energy barriers18,19 and may not be kinetically feasible in (1a+2H)+•. In contrast, we found that inter-ring proton migrations from the adenine cation to the C-8 position of its radical counterpart had low-energy barriers in TS1 and TS2 (Scheme 1, Table S3, Supporting Information), and the resulting cation radicals 6+• and 7+• were more stable than (1a+2H)+•. This makes the isomerizations substantially exothermic and kinetically feasible, as illustrated by the calculated rate constants shown in Figure S12 (Supporting Information). Isomerizations of the same type were also indicated by proton migration in (1c+2H)+• and (1b+2H)+•, forming new C-8 and, C-8″ protonated cation radicals 8+• and 9+•, respectively. The low-energy cation radical structures (1a+2H)+•, (1b +2H)+•, (1c+2H)+•, 6+•, 7+•, 8+•, and 9+• were examined by M06-2X and ωB97X-D/6-31+G(d,p) TD-DFT calculations to obtain UV−vis absorption spectra and compare them with the action spectrum of the (1+2H)+• ion. The absorption spectra of (1a+2H)+• and (1b+2H)+• (Figure S13, Supporting Informa-

Figure 2. Electron-transfer dissociation (ETD) mass spectra of (a) (1+2H)2+ and (b) (1+DBCE+2H)2+. The expanded mass regions of (1+2H)+• are from ETD of (c) (1+DBCE+2H)2+ and (d) (1+CE +2H)2+.

the protonation tautomers depended on the computational method used and were affected by solvation energies (Table S2, Figure S7, Supporting Information). The (1a+3′-CE+2H)2+ and (1c+3′-CE+2H)2+ complexes showed the lowest free energies, and both the 1,1″ and 3,1″ tautomers were selected for in silico formation and study of cation radicals. In contrast, whereas protonation of 2 favored positions 3 and 3″ to give the lowest-energy tautomer (2a+2H)2+ (Table S1), the 1,3″ protonated tautomer (2b+CE+2H)2+ was the lowest-energy ion among the CE complexes (Figure S8, Supporting Information). This indicates favorable solvation by the CE ligand of the H−N-1−C−NH2 moiety in the N-1- protonated tautomers. However, both the 1,3″ and 3,3″ tautomers of (2+2H)2+ were considered for further analysis. UV−Vis Action Spectrum of (1+2H)+•. The UV−vis photodissociation action spectrum of (1+2H)+• showed a broad band in a visible region with a maximum at 465 nm and two UV bands with maxima at 340 and 260 nm (Figure 3a). The main photodissociation channels were the loss of dehydrated deoxyadenosine (Δm = 233 Da), forming a cation radical fragment ion at m/z 333, and loss of a dideoxyadenosyl radical (Δm = 234 Da), forming the even-electron fragment ion at m/z 332. A minor channel was also observed leading to loss of adenine (m/z 431, Figure S9, Supporting Information). The 260 nm band in the action spectrum of (1+2H)+• (Figure 3a) is analogous to the main band in the action spectra of (1+H)+ and (1+2H)2+ in the same region of the spectrum (Figure S10a,b, Supporting Information) and consistent with the previously reported action spectrum of protonated adenine.25,30 Interpretation of the 340 and 465 nm bands required extensive 4102


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Figure 3. (a) UV−vis action spectrum of (1+2H)+• showing the sum of fragment ion intensities (black line) and mass-resolved photodissociation channels for m/z 333 and m/z 332 fragment ions. (b) UM06-2X/6-31+G(d,p) TD-DFT absorption spectrum of 6+• (bars) with a Lorentzian fit at 10 nm full width at half-maximum (fwhm, black); the blue line inset with red error bars shows the calculated spectrum including 330 vibronic transitions of the 12 lowest excited states at 300 K. UM06-2X/6-31+G(d,p) TD-DFT absorption spectra (bars) with a Lorentzian fit at 10 nm fwhm of (c) 7+•, (d) 8+•, and (e) 9+•. Band assignment: π = πz → πz* transitions; CT = charge-transfer transitions.

tion) gave a poor fit as they lacked an intense transition in the 300−360 nm region to match the prominent band in the action spectrum. The calculated spectrum of (1c+2H)+• had only a very weak transition at 450 nm to match the 460 nm band in the action spectrum and showed a transition at 685 nm that was absent in the action spectrum. Similar mismatches disqualified other, higher-energy (1+2H)+• isomers (Figure S13). The closest match with the action spectrum was obtained for lowenergy ions 6+•, 8+•, and 9+•, which showed most features displayed in the action spectrum (Figure 3b,d,e). The fit is particularly tight for the vibronically broadened absorption

spectrum of 6+• that was calculated for the 300−700 nm region (Figure 3b, inset). Hence, on the basis of both the spectra fit and low energy, ions 6+•, 8+•, and 9+• are likely to be the final stable products of electron attachment to dAA dications (1+2H)2+. We note that all of these structures have the 8,8− 2H position in the same nucleobase carrying the charging proton, which gives rise to the chromophore displaying the observed electronic transitions. The nature of the electronic excitations in 6+•, 8+•, and 9+• was assigned on the basis of molecular orbital analysis (Tables S4−S6, Supporting Information). The transitions at 445, 414, 4103


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Figure 4. ωB97X-D/6-31+G(d,p) optimized structures of (1a+2H)+•, (1b+2H)+•, and (1c+2H)+• showing atomic spin densities (purple italics) and charges (black numerals) summed for the 3′- and 5′-adenine ring atoms. Atom color coding is as in Figure 1.

The TD-DFT spectra of 8+• and 9+• chiefly consisted of nucleobase-localized excitations, whereas charge-transfer transitions were weak. The intense transitions at 402 and 304 nm in the spectrum of 8 +• (Figure 3d) originated from a combinations of πz → πz* electron excitations within the αand β-molecular orbital manifolds of the 8,8−2H-adenine cation radical, as shown in Figure S15 (Supporting Information). The intense transition at 233 nm was due to πz → πz* excitations within the protonated 3′-adenine ring. The TD-DFT spectrum of 9+• (Figure 3e) was similar, displaying πz → πz* transitions at 429 ad 306 nm and an intense line at 231 nm. This similarity is not unexpected with regard to the similar electronic structure of the main chromophores in 8+• and 9+•. In summary, TD-DFT analysis indicates that the bands in the UV−vis action spectrum of the (1+2H)+• cation radicals are chiefly due to πz → πz* excitations within the 8,8−2H-adenine cation radical moiety in low-energy ions 6+•, 8+•, and 9+•, with a possible contribution of isomer 7+•. Adenine Ribonucleotide Chimeras. UV−vis photodissociations of the chimeric ribonucleotides (2+2H) +• , (3+2H) +• , (4+2H) +•, and (5+2H) +• were similar, proceeding by elimination of (adenine+H) radicals (Δm = 136 Da) followed by the ribose residue (Δm = 114 Da; Figures S3−S6, Supporting Information). The UV−vis action spectrum of the p-phenylene chimera (2+2H)+• showed a broad band at 410− 530 nm and two intense bands centered at 340 and 250 nm (Figure 5a). The action spectrum of the 4,4′-biphenylene chimera (3+2H)+• was similar, although the long-wavelength 410−530 and 350 nm bands were relatively less intense and another band at 215 nm was distinct (Figure S16a, Supporting Information). Similar features were also apparent in the action spectrum of the 2,6-naphthalene chimera (4+2H)+• (Figure S16b). To interpret the data, we carried out TD-DFT calculations of excited states of several tautomers and conformers of (2+2H)+• and, to a limited extent, also (3+2H)+•. Upon electron attachment to the dications of the lowest-energy p-phenylene chimeras, (2a+2H)2+, (2b+2H)2+, and (2c+2H)2+, we obtained several isomeric cation radicals, ranging from fully extended structures through half-folded ones to fully folded ones. The relative energies highly favored the folded structures (Table S8, Supporting Information), indicating that electron attachment was likely to result in conformational collapse, as found for the dAAs. The UV−vis absorption spectra from TD-DFT calculations of the lowest-energy cation radicals (2a+2H)+•, (2aa+2H)+•, and (2ba+2H)+• displayed all of the important bands in the action spectrum of (2+2H)+• (Figure 5b−d). The closest match was obtained for (2ba+2H)+•, which showed

Scheme 1. Isomerizations of (1a+2H)+•, (1b+2H)+•, and (1c +2H)+• by Proton Transfer between the Adenine Rings

and 325 nm (labeled with red π in Figure 3b) in the spectrum of 6+• are chiefly produced by a πz → πz* electron excitation within the 3′-adenine cation radical moiety (see the molecular orbital analysis in Figure S14, Supporting Information). In contrast, the transitions at 369 and 284 nm (labeled CT in Figure 3b) are associated with inter-ring charge transfer by electron excitation from the singly occupied πz molecular orbital (SOMO, 148α) at 3′-adenine to vacant πz* orbitals on the 5′-adenine ring. The intense transition at 228 nm (labeled with black π) chiefly consists of excitations within the πz system of the neutral 5′-adenine ring. We note that the intensity of the CT transitions is likely to be exaggerated by TD-DFT calculations.32 Analysis of the equivalent excited states by configuration-interaction singles (CIS) calculations showed the CT states to have very low oscillator strength (Tables S6 and S7, Supporting Information), and therefore, their appearance in the TD-DFT spectra must be judged with caution. 4104


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positively charged protonated adenine moieties. These attractive interactions also provide a rationale for the exothermic conformational collapse upon electron attachment. The magnitude of the zwitterionic stabilization can be gauged by the calculated ion−electron recombination energies of the dications, which were |REadiab| = 5.34 eV for the protonated adenine ring in (2d+2H)2+ and |REadiab| = 6.39 eV for the pphenylene ring in (2a+2H)2+ (both ωB97X-D values). By comparison, the intrinsic adiabatic electron affinity (EA) of the terephthalate ring in neutral di-5′-ribosyl ester lacking the adenine rings was substantially lower, EA = 1.09 eV, which was similar to the experimental33 and our ωB97X-D-calculated EA of dimethyl terephthalate (0.824 and 0.863 eV, respectively). This allowed us to estimate the magnitude of Coulomb stabilization in (2ba+2H)+• as ΔEadiab = 1.09 − 6.39 eV = −5.3 eV. The nature of excited electronic states in (2ba+2H)+• was obtained from molecular orbital analysis (Figure S18, Supporting Information), and the states were assigned according to the type of electron excitation as πz → πz* (π), charge transfer (CT), and πz → Rydberg (R) in Figure 5a. The action spectra of the 4,4′-biphenylene and 2,6-naphthalene riboadenosine chimeras were interpreted in a similar fashion, as described in the Supporting Information (Figures S16a,b and S17a,b). The Coulomb stabilization of the zwitterionic structures for (2+2H)+• and, presumably, also for (3+2H)+• and (4+2H)+• is analogous to the effects governing DNA ionization. Ortiz and co-workers have shown that ionization of 2′-deoxyriboadenosine and guanosine 5′-phosphate anions occurred by electron detachment from the nucleobase moiety,34,35 contrasting the order of ionization energies of the isolated dihydrogen phosphate anion and free nucleobases. Similar effects of preferential nucleobase ionization in oligonucleotides have been assigned to effects of water solvation, stabilizing the phosphodiester anion and nucleobase cation radicals.36 Electron attachment to (2+2H)2+, (3+2H)2+, and (4+2H)2+ is an inverse, reductive process relative to DNA ionization, which is governed by the same relationship between the electronic properties of the initial state of the dication and final state of the reduced cation radical. The zwitterionic structures of (2+2H)+•, (3+2H)+•, and (4+2H)+•, corroborated by action spectra match, point to the dominant role of Coulomb stabilization in the absence of solvent effects. In an attempt to prevent zwitterion formation in the cation radicals while retaining spatial constraints to nucleobase interactions, we generated the riboadenosine trans-1,4-cyclohexanedicarboxylate chimera (5+2H)+• (Figure S6) in which the adenine cation and radical moieties are located on the opposite sides of the cyclohexane ring. The action spectrum of (5+2H)+• showed a very weak broad band at 460−510 nm, another band with a maximum at 340 nm, and major bands at 215−270 nm (Figure 6a). Several cation radical conformers of the 1,3″- and 3,3″-adenine diprotonated tautomers were obtained by TD-DFT, which showed relative energies within 25 kJ mol−1, depending on the DFT method (Table S10, Figure S19, Supporting Information). The calculated absorption spectra of the lowest-energy isomers (5a+2H)+• and (5b +2H)+• showed very weak bands at λ > 650 nm, consistent with the absence of absorption in the same region of the action spectrum, and a series of weak bands in the 400−480 nm region (Figure 6b,c). Multiple weak absorption bands in this region were also calculated for the other isomers (5c+2H)+•

Figure 5. (a) UV−vis action spectrum of (2+2H)+•; M06-2X/631+G(d,p) TD-DFT calculated absorption spectra of (b) (2ba+2H)+•, (c) (2aa+2H)+•, and (d) (2a+2H)+•. Insets show the ωB97X-D/631+G(d,p) optimized structures. Band assignment: π = πz → πz* transitions; R = πz → Rydberg transitions; CT = charge-transfer transitions.

major bands at 442, 313, and 241 nm and only very weak absorption bands above 550 nm (Figure 5b). We did not attempt to calculate vibronically broadened spectra for this larger system where performing hundreds of TD-DFT calculations of multiple excited states was beyond our computational resources. We note that the formation of (2ba +2H)+• is consistent with the preferred coordination of the CE ligand at the adenine N-1−H charged group in the precursor ion, leading to the formation of a N-1−H, N-3″−H tautomer of the cation radical upon electron-transfer dissociation (ETD). The nature of the ground electronic states in (2a+2H)+•, (2aa+2H)+•, and (2ba+2H)+• was indicated by the atomic charge and spin densities that placed the odd electron and negative charge in the terephthalate ester moiety while the protonated adenine rings carried negligible spin density and a substantial positive charge. Hence, the structures can be characterized as zwitterionic dication−anion radicals where the odd electron and negative charge in the phenylene spacer are stabilized by through-space Coulomb interactions with the 4105


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403 nm (ΔEexc = 3.07 eV) is from excitation from the SOMO to a Rydberg orbital delocalized over the reduced ring. In conclusion, UV−vis action spectroscopy in combination with electronic structure and excited-state calculations indicates that cation radicals of deoxyriboadenosine dinucleotide (dAA) and riboadenosine chimeras undergo a conformational collapse when formed by electron transfer to dications in the gas phase. The collapse in dAA is associated with exothermic proton transfer between the adenine nuclei targeting the C-8 position. The collapse in riboadenosine diester chimeras is facilitated by electron capture in the aromatic ring forming zwitterionic cation radicals with stacked adenine rings. Electron excitation in the cation radicals chiefly involves transitions of the πz → πz* and πz → Rydberg type within the adenine nucleus. Chargetransfer excitation between the adenine nuclei and aromatic rings gives rise to less prominent absorption bands.

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EXPERIMENTAL AND CALCULATIONS All compounds, materials, and procedures for ion formation by electrospray are described in the Supporting Information. UV− vis action spectroscopy measurements were carried out as reported previously31 and are described in the Supporting Information. Geometry optimizations, frequency calculations, TD-DFT, and kinetic calculations are described in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01856. Experimental and computational details and supplementary references, Tables S1−S3, S8, and S10 with calculated energies, Tables S4−S7, S9, and S11 with excited-state transitions, Figures S1−S21 with optimized structures, ETD and UVPD spectra, and molecular orbital analysis (PDF)

Figure 6. (a) UV−vis action spectrum of (5+2H)+•; TD-DFT M062X/6-31+G(d,p) absorption spectra of (b) (5b+2H)+• and (c) (5a +2H)+•. Band assignment is as in Figures 3 and 5.

through (5e+2H)+• to fit the weak 450−500 nm band in the action spectrum (Figure S20, Supporting Information). The broad band at 300−350 nm in the action spectrum can be assigned to absorptions in the same region of isomer (5b +2H)+•, whereas the other isomers had bands at λ < 290 nm. On the basis of spectral match, (5b+2H)+• appears to be the most likely isomer produced by ETD despite not being the lowest-energy structure according to DFT calculations. The charged and reduced adenine rings were in proximity in structures (5a+2H)+• and (5b+2H)+•, indicating attractive ion−dipole and ion-induced dipole interactions between the nucleobases. The inward bend of the adenine units across the cyclohexane ring is facilitated by strong hydrogen bonds of N3−H on the charged adenine to the ester carbonyl (Figure 6b,c). Interestingly, hydrogen bonding interactions of N-3−H resulted in lowering of the energy of the cyclohexane boat conformer (5e+2H)+• to within 13 kJ mol−1 of the most stable isomer (5a+2H)+• (Table S10, Figure S20c, Supporting Information). The nature of the electron transitions in (5b+2H)+• was investigated for some of the diagnostic absorption bands. The weak transition at 660 nm (ΔEexc = 1.88 eV) as well as that at 363 nm (ΔEexc = 3.41 eV) arises by electron promotion from the SOMO (MO177α) to the higher π molecular orbitals at the reduced adenine ring (Table S11, Figure S21, Supporting Information). The 299 nm transition (ΔEexc = 4.15 eV) corresponds to β-electron excitations to MO177β within the πorbital manifold on the reduced adenine ring. The weak 459 nm line (ΔEexc = 2.70 eV) is due to charge-transfer excitation from the SOMO on the reduced ring, and another weak line at

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 206-685-2041. Fax 206-685-8665. ORCID

František Tureček: 0000-0001-7321-7858 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the Chemistry Division of the National Science Foundation (Grants CHE-1359810 and CHE-1661815) and by the Klaus and Mary Ann Saegebarth Endowment is gratefully acknowledged.

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REFERENCES

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