The crystal structure of the orthorhombic form of L-(+)-histidine

L-(+)-Histidine (C6NaO2H9) crystallizes in the orthorhombic space group P212121, with a = 5-177, b = 7.322, c= 18.87 A, and Z=4. Data were collected with Mo Kct radiation, using balanced filters. The structure was solved by direct phasing methods and refined to a final agreement index of 0.034 for all reflections. The conformation of the molecule is that of the open, extended form, and is stabilized principally by an imramolecular hydrogen bond between the amino nitrogen atom and the adjacent imidazole nitrogen atom. Where this conformation is found in proteins, it is likely to reduce the chemical reactivity of tha+ ;midazole group, because one of the imidazole nitrogen atoms is sterically hindered by the peptide ba,.~, one.