Stereochemical criteria for polypeptides and proteins. V. Conformation of a system of three linked peptide units

The general conformations of a system of three linked peptide units are studied, and it is found that there are three types of conformations which contain NH…O hydrogen bonding between the first and the third units. One of them is part of a 3₁₀-helix, while the other two arc nonhelical. The two nonhelical conformations are very similar, and in both the cases the peptide chain turns around, reversing the direction of progress. Such a conformation can therefore occur in the region where a polypeptide chain folds back on itself, as in the cross-β structure. The method of representing these interesting tripeptide conformations in a (?,ψ) map is described. Examples of such hydrogen-bonded, nonhelical conformations which occur in peptides and proteins are discussed—e.g., in cyclohexaglyeyl, an open tetrapeptide Gly-L -Pro-L -Leu-Gly, and in parts of the lysozyme chain.