Stereochemical studies on cyclic peptides. VIII. Conformational analysis of hydrogen bonded cyclohexaglycyl molecule with a centre of inversion symmetry

Abstract

A study on the conformational aspects of cyclo-hexaglycyl having inversion symmetry has been made. The cyclic backbone has been assumed to have two internal 4→1 types of NH...O hydrogen bonds. This molecule has been found to take up two types of conformations designated as A^∗ and B^∗ having nearly the same energy values. The theoretical conformations have been compared with the conformations of cyclohexaglycyl hemihydrate observed in the crystal structure. Two molecules with an approximate inversion symmetry are close to the conformation of the type B^∗ and two other molecules with exact inversino symmetry correspond nearly to the types B^∗ and A^∗. comparison with the theoretically possible conformations of cyclohexaglycyl molecule with 2-fold symmetry has been made. The preference of inversion symmetry and preferred ranges of Ψ for glycyl molecules is discussed.