Effects of Methyl Side Chains on the Microsolvation Structure of Protonated Tripeptides.

The infrared predissociation spectra of the Gly3H+(H2O)1-2 and Ala3H+(H2O)1-2 clusters are presented and analyzed with the goal of revealing the influence of methyl side chains on the microsolvated structures of these flexible tripeptides. We have shown previously that the presence of methyl side chains can modulate the strengths of the intramolecular hydrogen bonds, thereby influencing the structures adopted by the bare tripeptides composed of glycine and alanine residues. This effect was attributed to the electron-donating nature of the methyl group, whose presence alters the proton affinities of the functional groups that are involved in hydrogen bonding. Here, we expand this work to the microsolvated tripeptides to determine how the effects of the presence of the methyl group evolve with the addition of water solvent molecules. For each solvated cluster, we found multiple solvated structures present, and their relative populations were disentangled using isomer-specific spectroscopic techniques and comparisons to calculation. The results showed that while the glycine and alanine tripeptides display similar structures for the dominant solvation population, they do have different structures for their minor solvation constituents stemming from their different bare tripeptide structures. The relative populations of these minor constituents indicate that the influence of the methyl side chain on intramolecular hydrogen bonding persists to some extent with solvation.

Conformational Changes Induced by Methyl Side-Chains in Protonated Tripeptides Containing Glycine and Alanine Residues.

We present a systematic study of the conformational and isomeric populations in gas-phase protonated tripeptides containing glycine and alanine residues using infrared predissociation spectroscopy of cryogenically cooled ions. Specifically, the protonated forms of Gly-Gly-Gly, Ala-Gly-Gly, Gly-Ala-Gly, Gly-Gly-Ala, Ala-Ala-Gly, Ala-Gly-Ala, Gly-Ala-Ala, and Ala-Ala-Ala allow us to sample all permutations of the methyl side-chain position, providing a comprehensive view of the effects of this simple side-chain on the 3-D structure of the peptide. The individual structural populations for all but one of these peptide species are determined via conformer-specific IR-IR double-resonance spectroscopy and comparison with electronic structure predictions. The observed structures can be classified into three main families defined by the protonation site and the number of internal hydrogen bonds. The relative contribution of each structural family is highly dependent on the exact amino acid sequence of the tripeptide. These observed changes in structural population can be rationalized in terms of the electron-donating effect of the methyl side-chain modulating the local proton affinities of the amine and various carbonyl groups in the tripeptide.

Direct Measurement of the Visible to UV Photodissociation Processes for the PhotoCORM TryptoCORM.

PhotoCORMs are light-triggered compounds that release CO for medical applications. Here, we apply laser spectroscopy in the gas phase to TryptoCORM, a known photoCORM that has been shown to destroy Escherichia coli upon visible-light activation. Our experiments allow us to map TryptoCORM's photochemistry across a wide wavelength range by using novel laser-interfaced mass spectrometry (LIMS). LIMS provides the intrinsic absorption spectrum of the photoCORM along with the production spectra of all of its ionic photoproducts for the first time. Importantly, the photoproduct spectra directly reveal the optimum wavelengths for maximizing CO ejection, and the extent to which CO ejection is compromised at redder wavelengths. A series of comparative studies were performed on TryptoCORM-CH3 CN which exists in dynamic equilibrium with TryptoCORM in solution. Our measurements allow us to conclude that the presence of the labile CH3 CN facilitates CO release over a wider wavelength range. This work demonstrates the potential of LIMS as a new methodology for assessing active agent release (e.g. CO, NO, H2 S) from light-activated prodrugs.

Competition between Solvation and Intramolecular Hydrogen-Bonding in Microsolvated Protonated Glycine and ß-Alanine.

Infrared predissociation (IRPD) spectroscopy is used to reveal and compare the microsolvation motifs of GlyH+(H2O)n and ß-AlaH+(H2O)n. The chemical structure of these amino acids differ only in the length of the carbon chain connecting the amine and carboxyl terminals, which nonetheless leads to a significant difference in the strength of the intramolecular C═H-N hydrogen bond in the unsolvated ions. This difference makes them useful in our studies of the competition between solvation and internal hydrogen bonding interactions. Analysis of the IRPD results reveals that the sequential addition of water molecules leads to similar effects on the intramolecular interaction in both GlyH+(H2O)n and ß-AlaH+(H2O)n. Solvation of the -NH3+ group leads to a weakening of the C═O···H-N hydrogen bond, while solvation of the carboxyl -OH leads to a strengthening of this bond. Additionally, we have found that for ß-AlaH+, the addition of a H2O to the second solvation shell can still influence the strength of the C═O···H-N hydrogen bonding interaction. Finally, because the C═O···H-N interaction in ß-AlaH+ is stronger than that in GlyH+, more solvent molecules are needed to sufficiently weaken the intramolecular hydrogen bond such that isomers without this bond begin to be energetically competitive; this occurs at n = 5 for ß-AlaH+ and n = 1 for GlyH+.

Microsolvation Structures of Protonated Glycine and l-Alanine.

The IR predissociation spectra of microsolvated glycine and l-alanine, GlyH+(H2O) n and AlaH+(H2O) n, n = 1-6, are presented. The assignments of the solvation structures are aided by H2O/D2O substitution, IR-IR double resonance spectroscopy, and computational efforts. The analysis reveals the water-amino acid as well as the water-water interactions, and the subtle effects of the methyl side chain in l-alanine on the solvation motif are also highlighted. The bare amino acids exhibit an intramolecular hydrogen bond between the protonated amine and carboxyl terminals. In the n = 1-2 clusters, the water molecules preferentially solvate the protonated amine group, and we observed differences in the relative isomer stabilities in the two amino acids due to electron donation from the methyl weakening the intramolecular hydrogen bond. The structures in the n = 3 clusters show a further preference for solvation of the carboxyl group in l-alanine. For n = 4-6 clusters, the solvation structure of the two amino acids is remarkably similar, with one dominant isomer present in each cluster size. The first solvation shell is completed at n = 4, evidenced by a lack of free NH and OH stretches on the amino acid, as well as the first observation of H2O-H2O interactions in the spectra of n = 5. Finally, we note that calculations at the density functional theory (DFT) level show excellent agreement with the experiment for the smaller clusters. However, when water-water interactions compete with water-amino acid interactions in the larger clusters, DFT results show greater disagreement with experiment when compared to MP2 results.

Probing Solvation-Induced Structural Changes in Conformationally Flexible Peptides: IR Spectroscopy of Gly3H+·(H2O).

IR predissociation spectroscopy of the Gly3H+(H2O) complex formed inside of a cryogenic ion trap reveals how the flexible model peptide structurally responds to solvation by a single water molecule. The resulting one-laser spectrum is quite congested, and the spectral analyses were assisted by both H2O/D2O substitution and IR-IR double resonance spectroscopy, revealing the presence of two contributing isomers and extensive anharmonic features. Comparisons to structures found via a systematic computational search identified the geometries of these two isomers. The major isomer, with all trans amide bonds and protonation on the terminal amine, represents ∼90% of the overall population. It noticeably differs from the unsolvated Gly3H+, which exists in two isomeric forms: one with a cis amide bond and the other with protonation on an amide C═O. These results indicate that interactions with just one water molecule can induce significant structural changes, i.e., cis- trans amide bond rotation and proton migration, even as the clustering occurs within an 80 K cryogenic ion trap. Calculations of the isomerization pathways further reveal that the binding energy of the water molecule provides sufficient internal energy to overcome the barriers for the observed structural changes, and the minor solvation isomer results from a small fraction of the ions being kinetically trapped along one of the pathways.

Accessing the Vibrational Signatures of Amino Acid Ions Embedded in Water Clusters.

We present an infrared predissociation (IRPD) study of microsolvated GlyH+(H2O) n and GlyH+(D2O) n clusters, formed inside of a cryogenic ion trap via condensation of H2O or D2O onto the protonated glycine ions. The resulting IRPD spectra, showing characteristic O-H and O-D stretches, indicate that H/D exchange reactions are quenched when the ion trap is held at 80 K, minimizing the presence of isotopomers. Comparisons of GlyH+(H2O) n and GlyH+(D2O) n spectra clearly highlight and distinguish the vibrational signatures of the water solvent molecules from those of the core GlyH+ ion, allowing for quick assessment of solvation structures. Without the aid of calculations, we can already infer solvation motifs and the presence of multiple conformations. The use of a cryogenic ion trap to cluster solvent molecules around ions of interest and control H/D exchange reactions is broadly applicable and should be extendable to studies of more complex peptidic ions in large solvated clusters.

IR-IR Conformation Specific Spectroscopy of Na+(Glucose) Adducts.

We report an IR-IR double resonance study of the structural landscape present in the Na+(glucose) complex. Our experimental approach involves minimal modifications to a typical IR predissociation setup, and can be carried out via ion-dip or isomer-burning methods, providing additional flexibility to suit different experimental needs. In the current study, the single-laser IR predissociation spectrum of Na+(glucose), which clearly indicates contributions from multiple structures, was experimentally disentangled to reveal the presence of three α-conformers and five ß-conformers. Comparisons with calculations show that these eight conformations correspond to the lowest energy gas-phase structures with distinctive Na+ coordination. Graphical Abstract ᅟ.