Conformation of cyclo (-L-Pro-Gly-) 3 and its Ca 2+ and Mg 2+ complexes

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Abstract
The synthetic hexapeptide cyclo(-L-Pro-Gly-)(3) is an ionophore that shows interesting conformational changes upon binding metal ions. X-ray crystallographic studies of this peptide show that when it is crystallized from an ethanol/ethyl acetate mixture the ring takes up an asymmetric conformation containing one cis peptide bond. In crystals of a Ca(2+) complex, the cation is sandwiched between two peptide molecules that differ markedly in conformation. However, both exhibit threefold symmetric forms, with all six peptide bonds in the molecule occurring in the usual trans conformation. The Ca(2+) is octahedrally surrounded by six glycyl carbonyl oxygens from the two peptides at an average distance of 2.26 A and can easily be released by the disruption of the peptide sandwich. In the magnesium complex, the peptide forms a 1:1 complex with the ion. The Mg(2+) is octahedrally coordinated to three glycyl carbonyls and three water oxygens. The average coordination distance between magnesium and the peptide oxygens is 2.03 A and that between magnesium and water oxygen is 2.11 A. The two peptide molecules in the asymmetric unit have similar conformations and have approximate threefold symmetry.