34967why proline is most commonly found in beta turns? In globular proteins nearly one-third of the amino acid residues are in turns orloops where the polypeptide chain reverses direction. Particularly common are βturns. The structure is a 180° turn involving four amino acid residues,with the carbonyl oxygen of the first residue forming a hydrogen bond with the amino-group hydrogen of thefourth. The peptide groups of the central two residues do not participate in any interresidue hydrogen bonding.Gly and Pro residues often occur in β turns, the former because it is small and flexible, the latter because peptide bonds involving the imino nitrogen ofproline readily assume the cis configuration, a form that is particularly amenable to a tight turn. Thus, proline introduce a sharp bend in the polypeptide chain The common presence of Pro and Gly residues in β turns and their relative absence in α helices, are readily explained by the constraints on the formation of the α helix. In proline, the nitrogen atom is part of a rigid ring, and rotation about the N -Cαbond is not possible. Thus, a Pro residue introduces a destabilizingkink in an α helix. In addition, the nitrogen atom of a Pro residue in peptide linkage has no substituent hydrogento participate in hydrogen bonds with other residues. For these reasons, proline is only rarely found withinan α helix.