OPTICAL RESOLUTION OF DL-HISTIDINE THROUGH INTERACTIONS WITH ACHIRAL GLYCOLIC ACID. Stephen Suresh and M. Vijayan, Molecular Biophysics Unit, Indian Institute of Science, Bangalore 560 012, India

As part of a programme aimed at studying biologically and evolutionarily important interaction and aggregation patterns, crystals of DL-histidine glycolate and L-histidine glycolate were prepared and analyzed. Crystallization experiments involving DL-histidine and glycolic acid yielded in addition to DL-histidine glycolate, a conglomerate containing crystals of L-histidine glycolate and D-histidine glycolate in an unusual process of chiral separation through interaction with an achiral molecule. The crystal structure of DL-histidine glycolate is made up of alternating layers of unlike molecules as in many other binary complexes involving amino acids. The arrangement of molecules in the structure of L-histidine glycolate is almost identical to that in one of the forms of L-histidine acetate, thus providing another example for the invariance of certain aggregation patterns with respect to changes in the molecules involved. The structure involves packing of columns containing L-histidine molecules and glycolate ions tightly hydrogen bonded to one another. In fact, among the six hydrogen bonds in the structure, five are between unlike molecules within the column. Thus, the histidine-glycolate interactions are much stronger than histidine-histidine interactions, while glycolate-glycolate interactions are non-existent. This points to the high propensity for the occurance of this type of aggregation involving chiral molecules in association with glycolate ions even in a solution containing molecules of both chiralities. Such aggregates might co-exist in high concentration with those involving both L and D histidine molecules and glycolate ions despite the entropic advantages the latter might have. The crystals obtained from the solution could then be those of L-histidine glycolate, D-histidine glycolate or DL-histidine glycolate, often leading to chiral separation. Thus the observed aggregation of molecules in the chiral complex also appears to provide a structural rationale for chiral separation of histidine in the presence of glycolic acid.