Structure and conformation of the calcium complex of cyclo(Ala-Leu-Pro-Gly)₂ in two crystal forms

Abstract

Crystal structures of two different forms of the calcium perchlorate complex of cyclo(Ala-Leu-Pro-Gly)₂ have been determined and refined using X-ray crystallographic techniques. Orthorhombic form: C₃₂H₅₂N₈O₈Ca(ClO₄)₂7H₂O.2CH₃OH, space group C222₁, a= 14.366, b= 18.653, c= 19.824 Ä, Z=4, R= 0.068 for 2208 observed reflections. Monoclinic form: C₃₂H₅₂N₈O₈Ca(ClO₄)₂.4H₂O, space group C2, a= 21.096, b= 10.182, c= 11.256 Å, β= 103.33°, Z= 2, R= 0.075 for 2165 observed reflections. The cyclic peptide molecule in both the structures has the form of a twofold symmetric, slightly elongated bowl. Type II'β-turns, involving Gly and Ala at the corners, exist at the two ends of the molecule. The interior of the molecule is substantially hydrophilic, and the external surface of the bowl is largely hydrophobic. The calcium ion is located at the centre of the mouth of the bowl-like molecule. In both crystal forms, four peptide carbonyl oxygens from the cyclic peptide and two solvent oxygens coordinate to the metal ion. The mode of complexation may be described as incomplete encapsulation as, for example, in the case of metal complexes of antamanide. In the crystal structures the complex ions are held together by hydrogen bonds involving perchlorate ions and water molecules. The molecular structure observed in the crystals is entirely consistent with the results of solution studies, which also indicate the conformation of the cyclic peptide in the complex to be similar to that of the uncomplexed molecule.