Conformational properties of Pro–Pro sequences. I. Crystal structures of two dipeptides with <scp>L</scp>‐Pro‐<scp>L</scp>‐Pro and <scp>L</scp>‐Pro‐<scp>D</scp>‐Pro sequences

Abstract

Abstract The crystal structures of Bu t CO‐ L ‐Pro‐ L ‐Pro‐NHMe, H 2 O ( 1 : monoclinic; P 2 1 ; a = 6.662, b = 11.067, c = 12.205 Å; β = 96.28°) and Bu t CO‐ L ‐Pro‐ D ‐Pro‐NHMe ( 2 : monoclinic; P 2 1 ; a = 10.770, b = 15.039, c = 11.325 Å; β = 110.00°) have been solved by x‐ray diffraction. Structure 1 accommodates an open disposition with intermolecular interactions involving the water molecule, while 2 is βII‐folded by an intramolecular i + 3 → i hydrogen bond. In both derivatives, small thermal parameters are indicative of fairly fixed conformations for the proline rings. Comparison between conformations of either isolated or adjacent L ‐Pro residues in the crystal structures of unstrained oligopeptides shows that the conformational properties of L ‐Pro‐ L ‐Pro sequences are probably a simple combination of those found for isolated L ‐Pro residues.