Crystal and molecular structure of L-(+)-histidine acetate dihydrate (at 123K)

Abstract

Abstract Single crystals of L-(+)-histidine acetate dihydrate were obtained from an aqueous solution containing L-histidine and acetic acid. The crystals are triclinic with the cell dimensions, a = 8.519(3) Å, b = 9.056(4) Å, c = 9.026(3) Å, α = 61.72(3)°, β = 86.63(3)°, γ = 86.26(3)°, space group = P1 and Z = 2. Intensities of 4725 reflections (MoK α were collected (at 123 K) using an automated four-circle diffractometer. The structure was solved using direct methods and refined to a final R index of 0.039 and Rw index of 0.048 for 4423 reflections with intensities larger than 3σ(I). Histidine acetate exists in the crystals as a salt where the proton from acetic acid is transfered to the imidazole ring whereas the carboxylic group is left unprotonized. The geometry of the two independent histidine molecules is nearly identical. There is an extensive system of hydrogen bonds stabilizing the structure.