Solid-state conformations of aminosuccinyl peptides: structure of tert-butyloxycarbonyl-L-prolyl-L-aminosuccinyl-glycyl-L-alanine methyl ester (Boc-L-Pro-L-Asu-Gly-L-Ala-OMe). A case of pseudo-translational symmetry.

C20H30N4O8, M(r) = 454.48, monoclinic, P2(1), a = 13.411 (2), b = 12.592 (2), c = 14.710 (1) A, beta = 104.30 (1) degrees, V = 2407 (6) A3, Z = 4, Dx = 1.254 Mg m-3, lambda (Cu K alpha) = 1.5418 A, mu = 0.783 mm-1, F(000) = 968, room temperature, final R = 0.086, wR = 0.080 for 4055 observed reflections. The title compound is a model for the intermediate formed in the deamidation reaction of porcine adrenocorticotropin hormone. The structure presents a pseudo-translational symmetry and was solved by using a modified version of the SIR88 package. In the refinement, few stereochemical restraints were needed to handle the static disorder shown by the C-terminal fragment of one molecule in the asymmetric unit. The conformation of the two independent molecules is almost identical and is a II' beta-bend, stabilized by an intramolecular hydrogen bond. In the crystal, screw-related molecules are linked by hydrogen bonds. The two molecules in the independent unit are related by the translation vector u = 0.4962 (2)a + 0.7310 (2)b + 0.5075 (2)c.