X-ray studies on crystalline complexes involving amino acids. I. Crystal structure of L-lysine L-aspartate

Abstract

Lysine aspartate, C 6 H 15 N 2 O 2 ·C 4 H 6 NO 4 , a crystalline complex or lysine and aspartic acid, crystallizes in the monoclinic space group P2 1 with two formula units in a unit cell of dimensions a = 5.539 (3), b = 7.848 (4), c = 15.324 (I5) A, β = 99.1 (I)°. The structure was solved by direct methods and Fourier techniques and refined to an R value of 0.069 for 1134 photographically observed reflexions. The dimensions and the conformation of the lysine molecule are similar to those observed in lysine mono-hydrochloride dihydrate. The side-chain carboxyl group of the aspartate ion is deprotonated unlike that in L- and DL-aspartic acid. The bond lengths and angles in the aspartate ion, except those in the side-chain carboxylate group, are comparable with those in L- and DL-aspartic acid. Considerable differences, however, exist in the conformation of the molecule in these structures. The crystal structure of lysine aspartate consists of alternating layers, one layer comprising lysine molecules and the other, aspartate ions. The two layers are interconnected primarily by hydrogen bonds between the side-chain amino group of lysine and carboxylate oxygen atoms belonging to neighbouring aspartate ions.