X-ray studies of crystalline complexes involving amino acids. III. The structure of the twinned pseudosymmetric crystals of a complex between histidine and aspartic acid

Abstract

L-Histidine-L-aspartic acid monohydrate, C₆H₉N₃0₂·C₄H₃NO₄·H₂O, crystallizes in the monoclinic space group P2_l, with eight formula units in a unit cell or dimensions a = 5.177 (3), b = 37.214 (20), c = 13.672 (10) Å, and β = 90°. The crystals are twinned about the a or the c axes leading to an apparent orthorhombic symmetry for the diffraction pattern. The X-ray analysis was further complicated by the presence or non-crystallographic 2₁ screw axes find a pseudo translation of c/2. The structure was solved in several stages using photographic data and refined to an R value of 0.113 using a modified least-squares procedure. The crystallographic asymmetric unit of each twin component contains four molecules each of histidine, aspartic acid and water. One half of these are related to the other half by non-crystallographic 2₁ screw axes parallel to the a axis. In each half, the two aspartic acid molecules (and the two water molecules) are related to each other by a pseudo translation of c/2; the pseudo translation is only approximate for the histidine molecules. Disorder in the stacking of layers, described by a translation of c/2, parallel to the b^∗ axis also exists in the structure leading to streaks in ure diffraction pattern along the row lines parallel 10 the b direction when I is odd. All the aspartic acid molecules in the structure have the same (and rather unusual) conformation with the side-chain carboxyl group gauche to both the α-amino and the α-carboxyl groups, Half the histidine molecules in the structure exist in an 'open' conformation whereas the other half exist in the 'closed' conformation. The crystal structure consists of alternating double layers, one double layer containing histidine molecules and the other aspartic acid molecules. The water molecules are sandwiched between the two layers in the aspartic acid double layer. The adjacent double layers are interconnected by hydrogen bonds between the amino and the imidazole groups of the histidine molecules on the one hand and the carboxyl groups of the aspartic acid molecules on the other.