X-ray studies on crystalline complexes involving amino acids and peptides. XXX. Structural invariance and optical resolution through interactions with an achiral molecule in the histidine complexes of glycolic acid

The crystals of DL-histidine glycolate and L-histidine glycolate were prepared and analysed as part of an ongoing programme aimed at studying biologically and evolutionarily important interaction and aggregation Patterns. 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 structure of L-histidine glycolate involves packing of columns containing L-histidine molecules and glycolate ions tightly hydrogen bonded to one another. The arrangement is almost identical to that in the structure of L-histidine acetate, thus providing another example for the invariance of certain aggregation patterns with respect to changes in the molecules involved. 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