Fundamental dimensions of polypeptide chains

Abstract The dimensions of the polypeptide chain and of its associated N—H•••O hydrogen bonds can be inferred with confidence. Data from many sources—X-ray diffraction analyses of crystals of organic acids, amides, peptides and related compounds; polarized infra-red studies of crystals—together with fundamental concepts of structural chemistry, now provide a basis for satisfactory knowledge and understanding of the dimensions and configurations of the amide group, C H / C'—N // C C The normal coplanarity of the atoms of this group is the result of resonance which gives rise to partial double-bond character of the N—C′ peptide bond. Rotation about this bond is, in general, severely restricted. The trans configuration of the amide group appears to be preferred. Interatomic distances and bond angles of the amide group which may be derived from the present data are: N—C = 1·47 Å C—N—C′ = 123° N—C′ = 1·32 C—N—H = 114 C′—O = 1·24 H—N—C′= 123 C′—C = 1·53 N—C′—O = 125 N—C′—C = 114 O—C′—C = 121 C′—C—N = 110 The dimensions and directional characteristics of N—H•••O hydrogen bonds have been determined in a great variety of crystals. In these crystals the N•••O distances vary, falling generally within the limits 2·79 ± 0·12 Ä. The H•••O vector rarely makes an angle greater than 20° with the probable direction of the N—H bond.