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The crystal structures of ButCO‐L‐Pro‐L‐Pro‐NHMe, H2O (1: monoclinic; P21; a = 6.662, b = 11.067, c = 12.205 Å; β = 96.28°) and ButCO‐L‐Pro‐D‐Pro‐NHMe (2: monoclinic; P21; 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.
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Aubry et al. (1985) studied this question.
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