X-ray studies of crystalline complexes involving amino acids. II. The crystal structure of L-arginine L-glutamate

Arginine glutamate monohydrate, C 6 H 15 N 4 O 2 ·C 5 H 8 NO 4 ·H 2 0, a crystalline complex of two amino acids, belongs to the orthorhombic space group P2 1 ,2 1 ,2 1 with four formula units in a unit cell of dimensions a = 5.047 (8), b = 9.849 (8), c = 32.500( 12) A. The structure was solved by the symbolic addition procedure and refined to an R value of 0.83 for 1007 photograhically observed reflexions. The conformation of the arginine molecule is different from those reported so far, whereas the conformation of the glumate ion is the same as that observed in the structure of glutamic acid hydrochloride. An analysis of the conformations or the two molecules found in crystal structures containing them is also presented. The crystal structure of the complex is stabilized by ionic interactions and hydrogen bonds. The crystal structure consists of alternating layers, one layer consisting of cationic arginine molecules and the other containing negatively charged glutamate ions. The adjacent layers are interconnected principally through a specific ion-pair interaction between the guanidyl group of arginine and the γ-carboxyl group of the glutamate, and a water bridge. The former which involves the electrostatic attraction between the positively charged guanidyl group and the negatively charged carboxylate group as well as two nearly parallel N-H ··· O hydrogen bonds, is the first such specific side chain-side chain interaction to be observed at atomic resolution.