Pyrrolidine ring puckering in cis and trans-proline residues in proteins and polypeptides. Different puckers are favoured in certain situations.

In a set of proteins studied at high resolution by X-ray crystallography over a half of all cis and trans-proline residues could be unambiguously assigned to one of the two forms of pyrrolidine ring puckering, called UP and DOWN. Of these, 89% of the cis-proline residues exhibit the DOWN pucker, while the trans-proline residues, on average, are about evenly distributed between the two forms. Of trans-proline residues found in alpha-helices, 79% have the UP ring pucker. trans-proline residues occurring in other situations are more equally distributed between the two forms of pucker, although further generalizations may be possible. Proline residues in a set of crystal structures of short polypeptides were also examined. As in the protein sample, a tendency for the cis-proline residues to have the DOWN pucker was observed, but the effect was less pronounced.