Cooperativity of multiple H-bonds in influencing structural and spectroscopic features of the peptide unit of proteins

Abstract

A glycine dipeptide is paired with one or more formamide molecules in a variety of different H-bonding configurations, monitoring the structural and spectroscopic features of the dipeptide via ab initio calculations. Of particular interest is the way in which the perturbations induced by a CH⋯O H-bond between the dipeptide and a proton acceptor are themselves affected by the presence of other H-bonds to the dipeptide. It is found that whether or not these other H-bonds are present, the introduction of a CH⋯O H-bond causes the C–H bond to shorten, and its stretching frequency is shifted to the blue. Also relatively unaffected is the NMR chemical shift of the CH proton which moves further downfield upon formation of the CH⋯O H-bond. In contrast, the effect of this CH⋯O H-bond upon the NH group of the dipeptide varies depending on whether or not there are other H-bonds present. Specific effects of cooperativity are less amenable to simple interpretation in that there are differences in behavior between the two conformations of the dipeptide examined.