Ionic co-crystals of enantiopure and racemic histidine with calcium halides

Abstract

Ionic co-crystals (ICCs) of L- and DL-histidine with CaCl₂, CaBr₂ and CaI₂ were prepared by mechanochemical and solution methods and were structurally characterized by either single crystal or powder X-ray diffraction methods. The L-histidine molecules bridge Ca²⁺ cations forming enantiopure ribbons in the homochiral crystals (L-His)₂·CaX₂·nH₂O (X = Cl and Br n = 3, X = I n = 4), as well as in the partial dehydration product of (L-His)₂·CaI₂·4H₂O, namely (L-His)₂·CaI₂·3H₂O. In the racemic (DL-His)₂·CaX₂·4H₂O cases (X = Cl, Br, X = I), molecules of both chiralities are accommodated in the coordination sphere of the Ca²⁺ cation forming ribbons with homochiral rims as in the enantiopure crystals. Intrinsic dissolution rate measurements show that the histidine-CaCl₂ co-crystals have a much higher IDR with respect to both enantiopure and racemic histidine solids.