Conformational analysis of Gly–Ala–NHMe in D₂O and DMSO solutions: a two-dimensional infrared spectroscopy study

A relevant number of experiments on short peptides has been performed in recent years. One of the major problems rises from the simultaneous presence of slightly different conformers at equilibrium in solution. In the present paper, the conformational characteristics of the Gly-L-Ala-Methyl amide dipeptide in D2O and DMSO solutions are investigated by nonlinear IR spectroscopy. The pump probe scheme with ultrashort midinfrared pulses, in the Amide I region, is used to determine the mutual orientation of the two C=O bonds and the dynamics due to solute solvent interactions. The coupling between Amide I modes is evaluated from both linear and 2D spectra. The interconversion between the different conformations occurs on time scales longer than the vibrational lifetime, and the spectral diffusion observed in 2D spectra is attributed to the solvent dynamics. Quantum mechanical calculations and molecular dynamics simulations are performed to identify the most stable geometries. By comparing the experimental and the theoretical data, we establish the prevalence of beta-like polar conformers in both water and DMSO solvents.