Effects of Solvent-Backbone Interactions on the Amide I Band of an Alanine-Rich Peptide

Abstract

Solvent conditions have long been influential in protein folding to functional three-dimensional conformations. We can directly measure the degree of intrapeptide and solvent-peptide hydrogen bonds through the use of infrared spectroscopy. Examination of the temperature-dependence of the amide I’ band can help elucidate the contributions of solvent-backbone interactions to the band position. Strong hydrogen bonding solvents like water shift the amide I bands to lower frequencies while weaker hydrogen bonding solvents, such as DMSO, shift the peaks to higher frequencies. In this work, we investigated AY6 (Ac-AAKAAY-NH2), an unstructured peptide, in D2O, 40% trifluoroethanol:D2O, and DMSO to probe both solvent and amide group effects. The backbone atoms in AY6 exhibit marked desolvation in DMSO, as evidenced by shifting of the amide I’ bands to higher frequencies compared to D2O. In addition, the slope of the plot of amide I’ frequency versus temperature is very similar in each solvent. We compare these findings to a series of other model systems.