CRYSTAL AND MOLECULAR-STRUCTURE OF L-(S)-HISTIDINE DIHYDROCHLORIDE (MONOCLINIC FORM)

Single crystals of L-histidine dihydrochloride were obtained from an a queous solution containing L-histidine and diluted hydrochloric acid. The crystals are monoclinic with the cell dimensions, a = 5.243(2) Ang strom, b = 7.257(1) Angstrom, c = 13.350(1) Angstrom, beta = 92.65(1)d egrees, space group P2(1) and Z = 2. Least-squares refinement using 26 85 X-ray (MoKalpha) reflections has led to an R factor of 0.029. The L -histidine moiety in this complex exists as a dication where the carbo xylic group as well as the amino nitrogen and the imidazole ring are p rotonated. Based on the N - C alpha - C beta - C gamma torsion angle ( psi(perpendicular to) = 57.1 degrees), the conformation of the histidi ne molecule in this structure is g(+), whereas the conformation in the orthorhombic form of this complex (reported earlier) is g(-). There a re no direct contacts (hydrogen bonds) between the histidine moieties as all the intermolecular bonding seem to involve Cl- ions.