X-ray studies on crystalline complexes involving amino acids and peptides. VIII. Head-to-tail arrangement and a specific interaction in the crystal structure of L-arginine acetate.

L-Arginine acetate crystallizes in the monoclinic space group P2(1) with a = 9.229(2), b = 5.178(3), c = 13.271(4) A and beta = 111.4(1) degrees. The crystal structure was solved by direct methods and refined to an R value of 0.058 for 1333 observed reflections. The conformation of the arginine molecules in the crystal is different from those observed in the crystals of L-arginine, its salts and complexes. The crystal structure indicates that complexation with a small carboxylic acid like acetic acid is sufficient to align arginine molecules in a head-to-tail fashion. The structure contains a specific ion-pair interaction, involving electrostatic attraction as well as two nearly parallel, N-H ... O hydrogen bonds, between the guanidyl group and the acetate ion.