Stable configurations of polypeptide chains

Several configurations of polypeptide chains involving planar amide groups with the dimensions found by experiment for simple substances, and hydrogen bonds between the NH groups and the carbonyl oxygen atoms, have been discovered. One of these structures, the $\alpha $-$\text{helix}$, with about 3.7 amino-acid residues per turn of the helix, has been assigned to synthetic polypeptides and proteins that give X-ray diagrams of the $\alpha $-$\text{keratin}$ type. The evidence supporting this assignment is reviewed. Other configurations of polypeptide chains, including the $\gamma $-$\text{helix}$, three pleated-sheet structures, and the three-chain helical structure proposed for collagen, are described, and evidence bearing on their possible presence in proteins is discussed.