Crystal structure of the 1:1 mixture of cyclic (L-Ala-L-Pro-L-Phe-L-Pro) and cyclic (L-Ala-L-Pro-D-Phe-L-Pro).

The conformation of the synthetic cyclic tetrapeptide Ala-Pro-Phe-Pro, C22H28N4O4, was established by X-ray diffraction methods. Although the synthesis was designed to produce only the LLLL isomer, the crystal structure analysis showed that the unit cell contained both the LLLL and LLDL isomers. The mixture crystallized in space group P2(1)2(1)2(1) with a = 20.532(7) A, b = 22.228 (9) A and c = 9.429 (2) A. The two independent molecules in the asymmetric unit were found to be diastereoisomers. Both molecules have a cis trans cis trans conformation for the backbone, however, the LLLL isomer has an approximate 2-fold rotation axis perpendicular to the average plane of the peptide ring, while the backbone in the LLDL isomer is quite asymmetric. Each of these conformations represents a new form, not reported previously.