An X-ray study of the hydrogen bonding in the crystalline L -arginine phosphate monohydrate complex

The crystal structure of the L-arginine phosphate monohydrate complex has been determined from 1453 three-dimensional X-ray diffractometer data and refined by least-squares methods to a residual of R = 3.9 %. The X-ray results suggest that the L-arginine molecule is deprotonated at the carboxyl group and protonated at the guanidyl and amino groups and that the phosphate ion carries one negative charge. The L-arginine molecule is in a less extended conformation than found previously and both the guanidinium group and the carboxyl-C system are essentially planar. The crystal structure is interlaced by an intricate hydrogen-bonding scheme with apparently one bifurcated hydrogen bond involving atom N ~. The symmetrical guanidinium-phosphate ion geometry proposed for the interaction of basic proteins with DNA is not found in the L-arginine-phosphate monohydrate structure. It may therefore be inferred that the symmetrical complex is not necessarily preferred over a more complicated hydrogen bonding arrangement. The same structure has been published recently by Aoki, Nagamo & Iitaka (Acta Cryst. (1971). B27, 11).