STRUCTURE AND CONFORMATION OF THE CALCIUM COMPLEX OF CYCLO(ALA-LEU-PRO-GLY)2 IN 2 CRYSTAL FORMS

Crystal structures of two different forms of the calcium perchlorate c omplex of cyclo(Ala-Leu-Pro-Gly)2 have been determined and refined usi ng X-ray crystallographic techniques. Orthorhombic form: C32H52N8O8.Ca (ClO4)2.7H2O.2CH3OH, space group C222(1), a = 14.366, b = 18.653, c = 19.824 angstrom, Z = 4, R = 0.068 for 2208 observed reflections. Monoc linic form: C32H52N8O8.Ca(ClO4)2.4H2O, space group C2, a = 21.096, b= 10.182, c = 11.256 angstrom, beta = 103.33-degrees, Z = 2, R = 0.075 f or 2165 observed reflections. The cyclic peptide molecule in both the structures has the form of a twofold symmetric, slightly elongated bow l. Type II' beta-turns, involving Gly and Ala at the corners, exist at the two ends of the molecule. The interior of the molecule is substan tially hydrophilic, and the external surface of the bowl is largely hy drophobic. The calcium ion is located at the centre of the mouth of th e bowl-like molecule. In both crystal forms, four peptide carbonyl oxy gens from the cyclic peptide and two solvent oxygens coordinate to the metal ion. The mode of complexation may be described as incomplete en capsulation as, for example, in the case of metal complexes of antaman ide. In the crystal structures the complex ions are held together by h ydrogen bonds involving perchlorate ions and water molecules. The mole cular structure observed in the crystals is entirely consistent with t he results of solution studies, which also indicate the conformation o f the cyclic peptide in the complex to be similar to that of the uncom plexed molecule.