Secondary and Tertiary Structure

The bond between the carbon atom of the carbonyl group and the amide nitrogen has partial double bond character that can be attributed to resonance: Accordingly, the carbonyl carbon and oxygen atoms, the amide nitrogen and hydrogen and the two adjacent α-carbon atoms He approximately in the same plane. Experimental evidence, mainly from x-ray crystallography, supports the near coplanarity of these six atoms in peptide backbones and also shows a relatively short distance between the carbonyl carbon and the nitrogen. The carbonyl oxygen and the amide hydrogen are on opposite sides of the (partial) double bond, at least in most peptide bonds. The cis arrangement is somewhat less stable and, since there is a considerable energy barrier between the cis and trans forms, the latter generally prevails.