Structural Dependence of NH Stretch Mode Frequency Shifts in Amide and Peptide Systems

Abstract

The NH stretch (s) frequency, ν(NH), in peptides and proteins, as in many amide group hydrogen bond (HB) systems, is used as an indicator of the presence and strength of the bond. This is based on the concept that ν(NH) is associated with an essentially localized mode whose frequency is influenced only by the HB interaction. We examine this assumption through ab initio studies of N-methylacetamide (NMA) and of 78 conformers of the alanine dipeptide (ADP). Based on our recent studies of nitrogen pyramidalization in NMA (J. Phys. Chem. A 2003, 107, 1825), we find that such nonplanarity, whether deriving from a nonplanar peptide group or only from a steric constraint, results in an increase in the NH bond length, r(NH), and thus in a decrease in ν(NH). This is accompanied by changes in other internal coordinates that make small contributions to this mode, emphasizing that although NH s is a highly localized mode it is not completely so. It is not surprising, therefore, to find that changes in the environment of the NH group, such as exist in the ADP conformers, lead to variations in ν(NH) even in the absence of traditional HB configurations. The ADP results permit an analysis of these effects and also illustrate the important role, as we recently emphasized (J. Phys. Chem. A 2002, 106, 11663), of electrical interactions in affecting r(NH) and thus ν(NH). We show how the increased strength of this interaction in ADP conformers with “standard” HBs leads, through the much larger induced dipole derivative, to the large increase in infrared intensity of NH s bands in such structures.