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

The infrared predissociation spectra of the Gly(3)H(+)(H2O)(1-2) and Ala(3)H(+)(H2O)(1-2) clusters are presentedand analyzed with the goal of revealing the influence of methyl sidechains on the microsolvated structures of these flexible tripeptides.We have shown previously that the presence of methyl side chains canmodulate the strengths of the intramolecular hydrogen bonds, therebyinfluencing the structures adopted by the bare tripeptides composedof glycine and alanine residues. This effect was attributed to theelectron-donating nature of the methyl group, whose presence altersthe proton affinities of the functional groups that are involved inhydrogen bonding. Here, we expand this work to the microsolvated tripeptidesto determine how the effects of the presence of the methyl group evolvewith the addition of water solvent molecules. For each solvated cluster,we found multiple solvated structures present, and their relativepopulations were disentangled using isomer-specific spectroscopictechniques and comparisons to calculation. The results showed thatwhile the glycine and alanine tripeptides display similar structuresfor the dominant solvation population, they do have different structuresfor their minor solvation constituents stemming from their differentbare tripeptide structures. The relative populations of these minorconstituents indicate that the influence of the methyl side chainon intramolecular hydrogen bonding persists to some extent with solvation.

Automated summary

The influence of methyl side chains on the microsolvated structures of Gly(3)H(+)(H2O)(1-2) and Ala(3)H(+)(H2O)(1-2) clusters was investigated using infrared predissociation spectra. Multiple solvated structures were observed for each cluster, and their relative populations were determined. The results showed that the presence of methyl side chains affects the intramolecular hydrogen bonding even in the presence of solvent molecules.