Article pubs.acs.org/JPCA
Proton-Bound 3‑Cyanophenylalanine Trimethylamine Clusters: Isomer-Specific Fragmentation Pathways and Evidence of Gas-Phase Zwitterions W. Scott Hopkins,*,† Rick A. Marta,† and Terry B. McMahon* Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *
ABSTRACT: The structures and dissociation pathways of the proton-bound 3-cyanophenylalanine·trimethylamine cluster have been studied using a combination of infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations. Three isomer motifs are identified: charge-solvated, zwitterionic, and trimethylamine (TMA)-bridged. While the TMA-bridged structures fragment to yield protonated TMA (channel 1) and protonated 3cyanophenylalanine (channel 2), charge-solvated species exclusively fragment via channel 1 and zwitterionic species exclusively fragment via channel 2. Mechanisms are proposed.
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INTRODUCTION It has been well established that aqueous amino acids exhibit a zwitterionic structure while, in the gas phase, only canonical structures are observed for individual amino acid molecules.1−8 In one rare instance a nontraditional zwitterionic form of arginine was identified whereby the guanidine side chain acted as the basic site, rather than the backbone amine of the amino acid.2 Interestingly, zwitterionic charge separation, which is unfavorable in the absence of solvation, has been shown in some cases to be stabilized by a single solvent molecule.1 Recent work from the McMahon laboratory at the University of Waterloo has explored under what conditions it might be possible to observe zwitterionic-like structures of amino acids in the gas phase, and several examples have been reported in which complexes of protonated amino acids with selected neutral molecules lead to structures resembling zwitterions.6,7 This previous work suggests that alkylated amines may prove especially good for stabilizing gas phase zwitterionic structures. Structural assignment of gas phase clusters containing amino acids is challenging owing to the very complex potential energy surfaces (PESs) that are typical of these systems. Oftentimes numerous conformational minima exist, and ensemble populations are spread across several of these structures. Even in low temperature sources (e.g., supersonic expansion, electrospray ionization) where clusters are rotationally and vibrationally cooled, a variety of conformations may be observed if barriers to isomerization are sufficiently high.5 This conformational trapping phenomenon has been observed for systems with barriers to interconversion as low as about 400 cm−1 (ca. 4.8 kJ/mol).9 As such, it is important that a thorough PES search be undertaken for spectroscopic studies of protonated amino acid clusters so as to identify all isomers that are potential spectral carriers. The basin-hopping search © 2013 American Chemical Society
strategy is commonly employed to identify low energy structures on complex PESs.10 The basin-hopping methodology may be viewed as a Monte Carlo with minimization technique wherein random structural distortions are accepted provided specified geometric and/or energetic criteria are met. In the present work, infrared multiple photon dissociation (IRMPD) has been used to record the vibrational spectrum of the protonated 3-cyanophenylalanine·trimethylamine cluster. The IRMPD technique is an established method for recording the vibrational spectra of gas-phase ions and clusters.11−15 Intriguingly, we find that different vibrational spectra are observed for the two dominant fragmentation channels; production of protonated trimethylamine (TMA·H+; m/z = 60 amu) and production of protonated 3-cyanophenylalanine (3-cyanophenylalanine·H+; m/z = 191 amu). These experimental findings are supported with a basin-hopping analysis and electronic structure calculations so as to elucidate which cluster structures give rise to the observed spectra and to deduce the origin of the distinct spectra observed in each mass channel.
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EXPERIMENTAL METHODS IRMPD experiments were carried out at the Centre de Laser Infrarouge d’Orsay (CLIO) free electron laser (FEL) facility at the University of Paris XI. The experimental apparatus has been described in detail previously.15−17 Stoichiometric quantities of trimethylamine hydrochloride (Sigma Aldrich) and 3-cyanophenylalanine (Alfa Aesar) were used to prepare 1 to 10 μM Received: August 3, 2013 Revised: September 17, 2013 Published: September 19, 2013 10714
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aqueous solutions. Chemicals were used without further purification. Gas phase proton-bound 3-cyanophenylalanine· TMA clusters were generated by positive mode electrospray ionization and were subsequently transferred to a Bruker Esquire 3000+ ion trap mass spectrometer where they were mass-selected, trapped, and irradiated by the tunable infrared output of the FEL. The FEL used electron energies of 46 MeV, allowing for continuous scans over a wavenumber range of 1000−2000 cm−1. Parent ion depletions and daughter ion enhancements were recorded as a function of FEL wavenumber to generate vibrational spectra of the mass selected ions.
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COMPUTATIONAL METHODS Owing to the complexity of the associated PES, the basinhopping surface sampling strategy was employed to identify low energy cluster structures.10 Structural sampling was undertaken using a custom-written routine that was interfaced with the Gaussian 09 suite for computational chemistry.18 TMA and 3cyanophenylalanine were modeled using the AMBER force field and partial charges that were calculated at the B3LYP/631+g(d,p) density functional theory (DFT) level for the isolated molecules using the CHelpG partition scheme.19−21 For each random step, TMA and the charge-carrying proton were given a random translational step size of −0.2 ≤ η ≤ 0.2 Å in each of the X, Y, and Z directions. TMA was also given a random internal rotation of −5° ≤ θ ≤ 5° about the TMA-fixed x, y, and z axes, and a random rotation of −5° ≤ φ ≤ 5° was applied about each of the dihedral angles along the unsaturated carbon chain of the substituted phenylalanine. In total, 10,000 basin-hopping steps were taken over four separate 2500 step PES searches. For each of these searches, a different preoptimized cluster structure was used as the initial input geometry. This treatment identified 136 cluster isomers, of which the 48 lowest-energy species (i.e., those within 30 kJ/mol of the predicted global minimum) were carried forward for optimization at the DFT level of theory. The 48 isomer test set carried forward from the molecular mechanics search reduced to 13 distinct cluster isomers following DFT optimization. These thirteen isomers are shown in Figure 1. Electronic structure calculations were undertaken using the B3LYP functional and a 6-311++G(d,p) basis set, as this has previously been demonstrated to be a suitable protocol for similar sized ionic systems.17,22,23 To ensure that each geometrically optimized species was a local minimum on the PES, normalmode analyses were undertaken, which also served to predict the harmonic vibrational spectra for each cluster isomer. For the lowest energy isomer in each of the three structural motifs (i.e., global minimum, isomer 4 and isomer 7) anharmonically corrected vibrational frequencies have also been calculated. The Supporting Information summarizes the DFT results and provides the optimized x, y, z coordinates for the various cluster isomers.
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Figure 1. Thirteen lowest-energy isomers of protonated 3-cyanophenylalanine·TMA. The global minimum and isomers 2 and 3 are extended charge-solvated structures, isomers 4, 5, 6, and 13 are zwitterionic structures, and isomers 7−12 are TMA-bridged structures. Relative energies are given in kJ/mol and include thermal corrections at 298 K. Calculations employed the B3LYP functional and 6-311+ +G(d,p) basis set.
Figure 2. (A) IRMPD spectrum observed in the m/z = 60 amu mass channel. Predicted harmonic vibrational spectra for (B) global minimum, (C) isomer 2, (D) isomer 3, (E) isomer 4, (F) isomer 5, (G) isomer 6, (H) isomer 7, (I) isomer 8, (J) isomer 9. Predicted spectra of charge solvated species are shown in red, zwitterionic species in blue, and TMA-bridged species in green. Calculations employed the B3LYP functional and 6-311++G(d,p) basis set.
RESULTS AND DISCUSSION
When recording the IRMPD spectrum for protonated 3cyanophenylalanine·TMA, photodissociation was observed to occur via two mass channels corresponding to production of protonated TMA (TMA·H+; m/z = 60 amu) or protonated 3cyanophenylalanine (m/z = 191 amu). Interestingly, a different IR spectrum was observed in each of the two mass channels. Figures 2 and 3 show the 1100−1850 cm−1 region of the IRMPD spectrum for protonated 3-cyanophenylalanine·TMA
when monitoring on the two photoproduct channels. It is immediately obvious when examining Figures 2 and 3 that there are signature peaks associated with each product channel. The protonated 3-cyanophenylalanine channel (Figure 3) exhibits peaks at 1677 cm−1, 1611 cm−1, and 1171 cm−1, which are not observed in the TMA·H+ channel. Similarly, the TMA·H+ channel (Figure 2) has a signature peak at 1747 cm−1 10715
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molecules by 8−12%.27 For this reason, harmonic frequencies derived from B3LYP/6-311++G(d,p) calculations are often scaled by 0.9679.28 Here we find that the predicted harmonic CO stretching frequencies are indeed slightly blue-shifted when compared with those observed experimentally. However, applying the typical scaling factor of 0.9679 would overcompensate for the differences between the observed and the calculated vibrational wavenumbers. Fortunately, introduction of a scaling factor was not needed for the interpretation of the observed IRMPD spectra, so we report unscaled harmonic frequencies here. Having established the spectral carriers associated with each mass channel, a more thorough interpretation of the IRMPD spectra is now possible. The diagnostic peak in the TMA·H+ mass channel at 1747 cm−1 is assigned to a concerted C−O−H angle bend and −OH stretching motion that brings the carboxylic acid proton into close proximity with the amine group, while at the same time a carbonyl stretch extends the −CO oxygen toward the TMA·H+ proton. A second signature peak for the charge-solvated species occurs at 1400 cm−1 in the TMA·H+ channel. This peak is assigned to a highly localized C−O−H bending vibration which brings the carboxylic acid proton into close proximity with the amine group. Note that both of these vibrational motions look very much like the beginning of an intramolecular proton transfer in the extended charge-solvated structures. The remaining peaks in the IRMPD spectrum recorded via the TMA·H+ channel are easily assigned to group vibrations in the TMA-bridged structures. The transition observed at 1799 cm−1 coincides with the localized carbonyl stretching mode of the bridgingTMA structures, while the peaks at 1503 cm−1 and 1461 cm−1 can be assigned to proton wagging/bending modes in the TMA·H+ moiety. Similar vibrational motions are also predicted for TMA·H+ moiety in the extended charge-solvated isomers and the charge separated isomers (zwitterions). However, the fact that these two transitions appear in both mass channels when the diagnostic peaks for the charge-solvated and zwitterion isomer subsets do not (vide infra) suggests that that the 1503 cm−1 and 1461 cm−1 transitions must be associated with the TMA-bridged isomer group. The assignment of the TMA·H+ proton wagging peaks is slightly less obvious than other spectral assignments owing to the relatively weak IR absorption cross sections predicted for these transitions for all isomers. Of course, as mentioned above, IRMPD spectral intensities are not based solely on IR cross sections, but also rely on coupling efficiency to dissociative product channels. It is thus not surprising that for the protonated 3-cyanophenylalanine·TMA system vibrational modes involving proton motion would couple efficiently to dissociative modes wherein the charge-carrying proton is retained by one of the two molecular fragments. The vibrational transitions at 1799 cm−1, 1503 cm−1, and 1461 cm−1 assigned to bridging-TMA isomers are common to the IRMPD spectra recorded via both product channels. The signature peaks observed in the IRMPD spectrum recorded via the protonated 3-cyanophenylalanine channel, on the other hand, are assigned to vibrations associated with low-lying zwitterionic isomers. The signature peak observed at 1677 cm−1 arises from a diagnostic, concerted vibrational motion in the zwitterionic species whereby a −NH3+ hydrogen stretch brings the zwitterionic proton into close proximity with one of the δ = −0.5 oxygen atoms. This motion is analogous to that associated with the 1747 cm−1 transition for the charge-solvated species
Figure 3. (A) IRMPD spectrum observed in the m/z = 191 amu mass channel. Predicted harmonic vibrational spectra for (B) global minimum, (C) isomer 2, (D) isomer 3, (E) isomer 4, (F) isomer 5, (G) isomer 6, (H) isomer 7, (I) isomer 8, (J) isomer 9. Predicted spectra of charge solvated species are shown in red, zwitterionic species in blue, and TMA-bridged species in green. Calculations employed the B3LYP functional and 6-311++G(d,p) basis set.
where the m/z = 191 amu channel shows no transition, and a significantly enhanced relative intensity for the peak at 1400 cm−1. Note that IRMPD spectral intensities are not attributed solely to IR cross sections, but also rely on coupling efficiency to dissociative product channels. For this reason, experimentally measured IRMPD transitions are oftentimes assigned based predominantly on predicted transition frequencies/wavenumbers.16,17,22−26 We adopt the same methodology here when comparing the experimental spectra to the predicted harmonic spectra in Figures 2 and 3. The fact that different IRMPD spectra are observed for the two mass channels is explained by our computational analysis. Within the set of distinct structures shown in Figure 1 three isomer motifs can be identified: (1) extended charge-solvated species containing two hydrogen bonds [global minimum, isomers 2 and 3], (2) zwitterionic isomers [isomers 4, 5, 6, and 13], and (3) compact structures wherein a protonated TMA molecule is oriented in a bridging fashion to the 3cyanophenylalanine amine and carbonyl functional groups [isomers 7−12]. Note that the isomers shown in Figure 1 are numerically labeled in order of increasing total energy and that a thermal correction to the total energy at 298 K has been applied. In comparing the predicted harmonic spectra we find that the wavenumber region associated with the carbonyl stretching modes (1675−1800 cm−1) is diagnostic of the isomer motifs present in the cluster sample. Charge-solvated species exhibit CO stretching modes in the 1725−1775 cm−1 wavenumber region, zwitterionic species in the 1675−1715 cm−1 region, and TMA-bridged structures in the 1780−1805 cm−1 region. These predictions accord well with the observed IRMPD spectra where three peaks are observed in the 1675− 1800 cm−1 region: 1677 cm−1 (assigned to zwitterionic species), 1747 cm−1 (assigned to charge-solvated species), and 1799 cm−1 (assigned to TMA-bridged species). It has been established that harmonic frequencies determined by DFT self consistent field (SCF) methods typically overestimate experimentally determined frequencies for covalently bound 10716
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observed via the TMA·H+ product channel. The transition observed at 1414 cm−1 in the m/z = 191 amu channel is assigned to a localized −NH3+ bending mode. This mode, too, is the zwitterion counterpart to the analogous motion at 1400 cm−1 in the extended charge-solvated structures. The assignments of the m/z = 191 amu signature peaks that are observed at 1611 cm−1 and 1171 cm−1 are somewhat less obvious. While it is tempting to assign these transitions to amine and hydroxyl bending modes in a TMA-bridging structure, the fact that these peaks are not observed in the m/z = 60 amu IRMPD spectrum suggests that such an assignment would be incorrect. Instead, the transitions at 1611 cm−1 and 1171 cm−1 seem more likely to be associated with the −NH3+ symmetric stretching (predicted at 1629 cm−1) and wagging (predicted at 1127 cm−1) motions of the zwitterionic isomers. Again, these assignments are not intuitive based on the predicted IR spectra owing to the relatively low predicted IR absorption cross sections. However, such an assignment is internally consistent with our data set wherein we find that the observed vibrational modes are associated with proton-transfer coordinates. It should be noted here that predicted vibrational motions associated with the charge-carrying proton are expected to be somewhat red-shifted from observation since the B3LYP functional has been shown to consistently underestimate hydrogen-bond strengths and overestimate hydrogen-bond distances.28−31 Moreover, vibrational motions of shared protons may require more rigorous anharmonic treatment to yield correct predictions.32 Thus, comparison of the experimental data with our predicted harmonic spectra serve only as a guideline for spectral assignment. The important considerations are that (1) carbonyl stretch assignments clearly identify spectral carriers and (2) vibrational motions of chargecarrying protons seem to couple most efficiently to dissociative modes thereby leading to relatively intense IRMPD signals. To provide additional computational support for this spectral interpretation, anharmonic frequency calculations have been undertaken for the lowest energy isomer associated with each structural motif. For these three test cases the predicted, anharmonically corrected spectra are consistent with the above spectral interpretation. The anharmonic calculation results and predicted spectra are available in the Supporting Information. Based on the spectral assignments, an interesting picture emerges with regard to the observed isomeric structures of protonated 3-cyanophenylalanine·TMA. While the TMAbridged structures apparently fragment via both of the observed product channels, extended charge-solvated isomers dissociate exclusively via the TMA·H+ channel and zwitterionic isomers fragment exclusively via the product channel that yields protonated 3-cyanophenylalanine. It should be noted that the observed photodissociation processes are not mode selective processes (e.g., photofragmentation of VCO2+),33−35 but are rather isomer-specif ic processes. Figure 4 schematically shows the relative energies of the lowest-energy isomers associated with the three structural motifs identified for protonated 3cyanophenylalanine·TMA. For the global minimum (charge solvated) isomer, the threshold to TMA·H+ production (109.1 kJ/mol) is overcome by IRMPD at much lower energies than the TMA production threshold (283.5 kJ/mol). Thermodynamically, TMA production can occur at much lower energies provided that a protonated 3-cyanophenylalanine zwitterion is also formed. However, for this to occur from the global minimum, isomerization to a zwitterionic structure (e.g., isomer 4) via a low-energy proton-transfer transition state (21.2 kJ/
Figure 4. Dissociation pathways for protonated 3-cyanophenylalanine· TMA as calculated at the B3LYP/6-311G++(d,p) level of theory with thermal corrections at 298 K applied. Values in parentheses give the electronic energies calculated at the MP2/aug-cc-pVTZ level of theory. Energies are reported in kJ/mol. Charge-solvated and zwitterionic phenylalanine derivatives are represented by cs/zw-Phe.
mol) must first occur. Note that this isomerization pathway is also supported with calculations at the MP2/aug-cc-pVTZ level of theory (results shown in parentheses in Figure 4). Owing to the fact that IRMPD occurs via isomer-specific vibrational excitations, such an isomerization at the 1-photon level would render the molecule transparent to subsequent IR excitation at the given (global minimum selective) wavelength. This explains why the global minimum IRMPD spectrum is observed only in the TMA·H+ product channel. Similar logic based on the calculated thresholds shown in Figure 4 also explains why the IRMPD spectrum for the zwitterionic isomer is only observed in the m/z = 191 amu product channel. For the bridging-TMA isomer, the proton can easily be either retained or transferred to the amine moiety during the dissociation process to access either of the two low-energy product thresholds.
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CONCLUSIONS IRMPD of the protonated 3-cyanophenylalanine·TMA cluster results in distinct spectra as recorded via the protonated TMA (m/z = 60 amu) and protonated 3-cyanophenylalanine (m/z = 191 amu) fragment mass channels. A thorough search of the cluster PES reveals numerous isomeric forms which can be classified into three structural motifs: (1) charge-solvated, (2) zwitterionic, and (3) TMA-bridged. While TMA-bridged structures access both product channels, charge-solvated species fragment exclusively via the TMA·H+ product channel and zwitterionic species exclusively via the 3-cyanophenylalanine·H+ product channel. TMA·H+ can be produced via IRMPD of the global minimum (charge-solvated) isomer by absorption of seven ν̅ ≈ 1700 cm−1 photons. The lowest energy method for production of 3-cyanophenylalanine·H+ from the global minimum structure requires ten ν̅ ≈ 1700 cm−1 photons and, importantly, must proceed via isomerization to the lowestenergy zwitterionic structure. Direct dissociation of the global minimum to yield a charge-solvated 3-cyanophenylalanine·H+ requires an additional four ν̅ ≈ 1700 cm−1 photons, that is, twice the photon requirement for production of TMA·H+ from the global minimum charge-solvated structure. The product channel leading to TMA·H+ and 3-cyanophenylalanine zwitterion formation occurs at much higher energies (ca. 1011 kJ/mol; fifty ν̅ ≈ 1700 cm−1 photons) owing to the instability of gas phase zwitterionic amino acids. 10717
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(11) Asmis, K. R.; Pivonka, N. L.; Santambrogio, G.; Brummer, M.; Kaposta, C.; Neumark, D. M.; Woste, L. Science 2003, 299, 1375. (12) Dunbar, R. C.; Steill, J. D.; Polfer, N. C.; Berden, G.; Oomens, J. Angew. Chem., Int. Ed. 2012, 51, 4591. (13) Gruene, P.; Rayner, D. M.; Redlich, B.; van der Meer, A. F. G.; Lyon, J. T.; Meijer, G.; Fielicke, A. Science 2008, 321, 674. (14) Hamilton, S. M.; Hopkins, W. S.; Harding, D. J.; Walsh, T. R.; Gruene, P.; Haertelt, M.; Fielicke, A.; Meijer, G.; Mackenzie, S. R. J. Am. Chem. Soc. 2010, 132, 1448. (15) Martens, J. K.; Compagnon, I.; Nicol, E.; McMahon, T. B.; Clavaguera, C.; Ohanessian, G. J. Phys. Chem. Lett. 2012, 3, 3320. (16) Wu, R. H.; Marta, R. A.; Martens, J. K.; Eldridge, K. R.; McMahon, T. B. J. Am. Soc. Mass Spectrom. 2011, 22, 1651. (17) Ziegler, B. E.; Marta, R. A.; Martens, S. M.; Martens, J. K.; McMahon, T. B. Int. J. Mass Spectrom. 2012, 316, 117. (18) Frisch, M. J.; Trucks, G. W., et al. Gaussian 09, Revision A.02.; Gaussian, Inc.: Wallingford, CT, 2009. (19) Pearlman, D. A.; Case, D. A.; Caldwell, J. W.; Ross, W. S.; Cheatham, T. E.; Debolt, S.; Ferguson, D.; Seibel, G.; Kollman, P. Comput. Phys. Commun. 1995, 91, 1. (20) Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. J. Comput. Chem. 2004, 25, 1157. (21) Wiberg, K. B.; Rablen, P. R. J. Comput. Chem. 1993, 14, 1504. (22) Dunbar, R. C.; Steill, J. D.; Oomens, J. J. Am. Chem. Soc. 2011, 133, 1212. (23) Martens, S. M.; Marta, R. A.; Martens, J. K.; McMahon, T. B. J. Phys. Chem. A 2011, 115, 9837. (24) Crampton, K. T.; Rathur, A. I.; Nei, Y. W.; Berden, G.; Oomens, J.; Rodgers, M. T. J. Am. Soc. Mass Spectrom. 2012, 23, 1469. (25) Martens, S. M.; Marta, R. A.; Martens, J. K.; McMahon, T. B. J. Am. Soc. Mass Spectrom. 2012, 23, 1697. (26) Nieckarz, R. J.; Oomens, J.; Berden, G.; Sagulenko, P.; Zenobi, R. Phys. Chem. Chem. Phys. 2013, 15, 5049. (27) Clabo, D. A.; Allen, W. D.; Remington, R. B.; Yamaguchi, Y.; Schaefer, H. F. Chem. Phys. 1988, 123, 187. (28) Andersson, M. P.; Uvdal, P. J. Phys. Chem. A 2005, 109, 2937. (29) Ireta, J.; Neugebauer, J.; Scheffler, M. J. Phys. Chem. A 2004, 108, 5692. (30) Santra, B.; Michaelides, A.; Scheffler, M. J. Chem. Phys. 2007, 127, 184104. (31) van der Wijst, T.; Guerra, C. F.; Swart, M.; Bickelhaupt, F. M. Chem. Phys. Lett. 2006, 426, 415. (32) Headrick, J. M.; Diken, E. G.; Walters, R. S.; Hammer, N. I.; Christie, R. A.; Cui, J.; Myshakin, E. M.; Duncan, M. A.; Johnson, M. A.; Jordan, K. D. Science 2005, 308, 1765. (33) Lessen, D. E.; Asher, R. L.; Brucat, P. J. J. Chem. Phys. 1991, 95, 1414. (34) Citir, M.; Altinay, G.; Metz, R. B. J. Phys. Chem. A 2006, 110, 5051. (35) Citir, M.; Metz, R. B. J. Chem. Phys. 2008, 128, 024307.
The energetic arguments outlined above have important bearing on the observed IRMPD fragmentation patterns. Clearly, charge-solvated clusters will have a propensity for TMA·H+ production, and zwitterionic clusters will have a propensity for 3-cyanophenylalanine·H+ production. Of course, either of these structural motifs could access both product channels should a photoexcitation/isomerization/photodissociation mechanism occur. Owing to the distinct vibrational spectra exhibited by the charge-solvated and zwitterionic structures, however, photoexcitation of one structural motif followed by isomerization to the other would render those clusters transparent to subsequent photodissociation. As a final comment, the isomer-specific IR initiated photochemistry reported here seems not to be restricted to the protonated 3-cyanophenylalanine·TMA cluster. We have recently observed similar photoinitiated chemistry for the protonated 3-trifluoromethylphenylalanine·TMA cluster, thus suggesting that protonated TMA may be especially good for stabilizing gas-phase zwitterions. A comprehensive computational study of this system is also currently underway and will be reported in a future publication.
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ASSOCIATED CONTENT
S Supporting Information *
Protonated 3-cyanophenylalanine·TMA DFT results summaries and optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (W.S.H.). *E-mail:
[email protected] (T.B.M.). Author Contributions †
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the Natural Sciences and Engineering Research Council (NSERC) of Canada. We are also grateful to the Centre Laser Infrarouge d’Orsay (CLIO) team and technical support staff for their valuable assistance and kind hospitality. Furthermore, we would like to acknowledge high performance computing support from the SHARCNET consortium of Compute Canada.
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