Cooperativity in Amide Hydrogen Bonding Chains. A Comparison between Vibrational Coupling through Hydrogen Bonds and Covalent Bonds. Implications for Peptide Vibrational Spectra

Abstract

Vibrational frequencies of the coupled N H, C O stretches (amide I), and C N stretch/CNH bend (amide II) have been calculated by density functional theory (DFT) at the B3LYP/D95** level and compared for polyglycines in β-strands and chains of H-bonding formamides. The amide groups of the polyglycines are connected by covalent bonds but not by extended H-bonding interactions. Conversely, the H-bonding chains are connected by an extended H-bonding interaction, but not by covalent bonds. All three types of vibrations couple more strongly and are more red (amide I and N H) or blue shifted (amide II) in the formamide chains than in polyglycine. In contrast to polycglycine chains, the C O's nearest the center of the formamide are elongated compared to those near the ends. As a result, the C O's near the center of the chain of 10 formamides no longer couple effectively with the C O's near the ends. Isotopic substitutions of 13 C, 14 C, 18 O, and 2 H (deuterium) at individual sites allowed us to probe the natural frequencies of individual C O's and N H's. The central 13 C O's and 14 C O's of the formamide chains (but not polyglycine) are significantly more shifted than those at the ends from those of a model containing only one C O. The greatest intensity for the H-bonded C O's comes from the lowest frequency fully delocalized stretch. 14 C isotopic substitution confines the vibrations to either side of the position of substitution, effectively restricting the extent of delocalization and lowering the intensity of the vibration. In general, the vibrations studied couple more effectively through the H-bonds than through the covalent bonds.