PRIMARY STRUCTURE OF PROTEINS: Peptide -C=N- bonds are unable to rotate freely because of their partial double-bond character. So, the six atoms of the peptide bond lie in a single plane, with the oxygen atom of the carbonyl group and the hydrogen atom of the amide nitrogen trans to each other. A peptide bond is rigid and planar. The polypeptide chain contains a series of rigid planes with consecutive planes sharing a common point of rotation at α-C. By convention, the bond angles resulting from rotations are labelled ф(phi) for the N-Cα bond and ѱ(psi) for the Cα-C bond. Allowed values for and are graphically revealed when is plotted versus in a Ramachandran plot.