Conformational changes in cyclic (D‐Phe‐Pro‐Gly‐D‐Ala‐Pro) upon complexation with Mg++.

Upon long standing and very slow evaporation, a solution of the cyclic pentapeptide and Mg(SCN)2 in CDCl3/CD3CN produced colorless crystals of a 1:1 complex of the peptide and Mg++. X-ray diffraction analysis showed the contents of the asymmetric unit to be C24H31N5O5 X Mg++ X NCS- X HSO4- X 6H2O (P2(1)2(1)2(1), a = 8.444 (1) A, b = 10.735 (2) A, c = 39.726 (6) A). This first example of a metal ion complex with a cyclic pentapeptide is in the form of an infinite stack with alternating peptide and Mg++ moieties. The octahedrally coordinated Mg++ makes only two ligands with the peptide carbonyls: O(3)-Mg++ above and O(1)-Mg++ below. The equatorial coordinations involve three H2O molecules and NCS-. There is a profound change in conformation upon complexation, with several phi and psi values changing by 90 degrees to 230 degrees. One NCS- oxidizes to HSO4- to provide the counter-ion.