Fourth polymorph of [Phe4 Val6] antamanide (pentahydrate).

The cyclic decapeptide antamanide and the synthetic, biologically active analog [Phe4 Val6]antamanide (cyclic[ValProProPhePhe]2) crystallize in various crystal forms as a function of the solvent. The present crystalline polymorph obtained from acetone/water (also from ethanol/water and DMSO/water) crystallizes in space group P2(1)2(1)2(1) with a = 20.194 (30) A, b = 21.118 (31) A, c = 16,132 (25) A and four molecules of peptide in the unit cell. There are five cocrystallized water molecules per peptide molecule, of which four water molecules are intrinsic to the peptide molecule. Although the molecular packing is entirely different in each of the polymorphs, the conformation of the peptide molecule, including the intrinsic water molecules, is very similar in all the polymorphs.