Description:
<jats:title>Abstract</jats:title><jats:p>The interplay between side‐chain and main‐chain conformations is a distinctive characteristic of proline residues. Here we report the results of a statistical analysis of proline conformations using a large protein database. In particular, we found that proline residues with the preceding peptide bond in the <jats:italic>cis</jats:italic> state preferentially adopt a down puckering. Indeed, out of 178 <jats:italic>cis</jats:italic> proline residues, as many as 145 (81%) are down. By analyzing the 1–4 and 1–5 nonbonding distances between backbone atoms, we provide a structural explanation for the observed trend. The observed correlation between proline puckering and peptide bond conformation suggests a new mechanism to explain the reported shift of the <jats:italic>cis‐trans</jats:italic> equilibrium in proline derivatives. The implications of these results for the current models of collagen stability are also discussed.</jats:p>