Ionic Cocrystals of Racemic and Enantiopure Histidine: An Intriguing Case of Homochiral Preference

Abstract

Ionic cocrystals (ICC) of l- and dl-histidine with lithium halides (LiCl, LiBr, LiI) have been prepared by solid state and solution methods, and structurally characterized in order to compare the behavior of enantiopure and racemic crystals in the interaction with inorganic salts. It has been shown that the lithium cations interact selectively with enantiomers of one handedness only, to the extent that the crystals obtained with racemic dl-histidine can be described as a special type of cocrystals made of enantiopure l-histidine and d-histidine chains of the same type as those obtained with enantiopure l-histidine. This chiral preference is even more noticeable in the ICC obtained from dl-histidine and LiI, which is actually a conglomerate of l- and d-ICCs. It is also reported that attempts to prepare ICCs with other alkali/alkaline earth halides invariably yield crystals of the less stable polymorph of l-histidine.