Pressure effect on the amide I frequency of the solvatedα-helical structure in water

Abstract

As a model system of the pressure dependence of the amide I mode of the solvatedα-helical structure in a helical peptide, we have calculated the frequencyshifts of the amide I modes as a function of the distance betweentrans-N-methylacetamide (t-NMA) dimer and a water molecule () by the density-functional theory (DFT) method at theB3LYP/6−31G++(d,p) level. Two amide Ifrequencies at 1652 and 1700 cm−1 were observed under this calculation. The former is ascribed to the amideI mode forming the intermolecular hydrogen bond (H-bond) betweent-NMAand H2O in addition to the intermolecular H-bond in thet-NMA dimer. The latter is due to the amide I mode forming only the intermolecular H-bond in thet-NMA dimer. We have found that the amide I frequency at1652 cm−1 shifts to a lower frequency with decreasing (i.e., increasing pressure), whereas that at1700 cm−1 shifts to a higher frequency. The amide I frequency shift of1652 cm−1 is largerthan that of 1700 cm−1 by the intermolecular H-bond. Thus, our results clearly indicatethat the pressure-induced amide I frequency shift of the solvatedα-helical structure correlates with the change in .