X-ray studies on crystalline complexes involving amino acids and peptides. XXVI. Crystal structures of two forms of L-histidine acetate and a comparative study of the amino acid complexes of acetic acid.

L-Histidine acetate crystallizes in two forms: (I) orthorhombic; P2(1)2(1)2(1); a = 5.027, b = 11.126, c = 17.473 A; Z = 4; (II) monoclinic; C2; a = 15.649, b = 9.276, c = 8.566 A; beta = 94.65 degrees; Z = 4. The structures were solved by direct methods and refined to R-values of 0.056 and 0.089 for 1131 and 1330 observed reflections, respectively. The conformations of the histidine molecule in the two forms are different. However, both are such that they facilitate the occurrence of a specific interaction of the histidine molecule with a carboxylate group. The basic elements of aggregation are hydrogen-bonded histidine ribbons, but they are of different types in the two structures. The ribbons are interconnected by acetate ions to form the crystals. The structures contain two characteristic interaction patterns involving amino and carboxylate groups, one of which is observed for the first time. The two water molecules in form II and their symmetry equivalents form an uninterrupted hydrogen-bonded chain running through the crystal. They also present an interesting case of disorder in hydrogen bonds. A comparative study involving amino acid complexes of acetic acid shows that the presence of acetate ion could lead to new aggregation patterns, specific interactions and characteristic interaction patterns with varying degrees of similarity with those observed in other structures containing amino acids.