Configurations of Polypeptide Chains with Equivalent Cis Amide Groups

Abstract

In recent papers [1-3] we have pointed out that stable configurations of polypeptide chains must show the bond distances and bond angles found in simple peptides and related simple substances, that each amide group must be planar, with either the cis-configuration or the trans-configuration about the C'-N bond, and that the carbonyl and imino groups (except for proline or hydroxyproline residues) must be involved in the formation of hydrogen bonds, with N-H•••O distance approximately 2.8 A, and with the oxygen atom nearly on the N-H axis. We have also presented evidence indicating that certain configurations around the two single bonds from the amide groups to the a carbon atom are energetically favored, and have discussed the 36 polypeptide-chain structures involving equivalent trans-amide groups with one or another of six favored azimuthal orientations about the N-C single bond and also one of six favored orientations about the C-C' single bond. [4] In the present paper we discuss the 36 configurations of equivalent cis-amide groups corresponding to these favored orientations, and also a few other configurations of cis-amide groups.