Crystal and molecular structure of L-(S)-histidine dihydrochloride (monoclinic form)

Abstract

Abstract Single crystals of L-histidine dihydrochloride were obtained from an aqueous solution containing L-histidine and diluted hydrochloric acid. The crystals are monoclinic with the cell dimensions, a = 5.243(2) Å, b = 7.257(1) Å, c = 13.350(1) Å, β = 92.65(1)°, space group P21, and Z = 2. Least-squares refinement using 2685 X-ray (MoK α ) reflections has led to an R factor of 0.029. The L-histidine moiety in this complex exists as a dication where the carboxylic group as well as the amino nitrogen and the imidazole ring are protonated. Based on the N–Cα–Cβ–Cγ torsion angle (ψ ⊥ = 57.1°), the conformation of the histidine molecule in this structure is g+, whereas the conformation in the orthorhombic form of this complex (reported earlier) is g−. There are no direct contacts (hydrogen bonds) between the histidine moieties as all the intermolecular bonding seem to involve Cl− ions.