Conformational properties of Pro-Pro sequences. I. Crystal structures of two dipeptides withL-Pro-L-Pro andL-Pro-D-Pro sequences

The crystal structures of ButCO-L-Pro-L-Pro-NHMe, H2O (1: monoclinic; P21; a = 6.662, b = 11.067, c = 12.205 A; β = 96.28°) and ButCO-L-Pro-D-Pro-NHMe (2: monoclinic; P21; a = 10.770, b = 15.039, c = 11.325 A; 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.