Preferred proline puckerings in cis and trans peptide groups: Implications for collagen stability

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Abstract
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 cis state preferentially adopt a down puckering. Indeed, out of 178 cis 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 cis‐trans equilibrium in proline derivatives. The implications of these results for the current models of collagen stability are also discussed.