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.