Stereochemical criteria for polypeptide and protein chain conformations. VII. Effect of non-planarity and bond angle distortion at the α-carbon atom on the contact map for a pair of peptide units

As a first step in the study of the effect of the distortions of the angle NC<SUP>a</SUP>C'(τ) at the α-carbon atom and the planarity of the peptide unit (measured by parameter ω) on the steric map of two linked peptide units, these distortions have been analysed from known crystal structure data on amino acids, peptides and the globular proteins, myoglobin, lysozyme, α-chymotrypsin, carboxypeptidase-A, and ribonuclease-S. Corresponding to three different values each of τ and ω around the ideal values (of 110° and 180° respectively) the contact maps are drawn. Most of the observed conformations of non-glycyl residues in the globular proteins are found to be within the allowed regions. The 3→1 type of N-H. O=C hydrogen bond between adjacent units is considered and the possibility of the formation of this hydrogen bond around φ=+90° and φ=−90° (leading to a left-handed helix, if continued) is explored in more detail. The conformations at some of the residues in α-chymotrypsin and carboxypeptidase-A are shown to form the right-handed 3→1 type of hydrogen bonds