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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.

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There is no other hydrogen bonding in the structure. Instead the molecules are held together largely by van der Waal's forces between the methyl and f-butyl parts of the azetidinium ions, and by similar forces between the methyl groups of the methanesulfonate ions.The S-O distances in the methanesulfonate group are 1.438 and 1.439 Á, identical within experimental error. Cesium methanesulfonate38 has two different S-O distances, 1.434 and 1.485 A, but Brandon and Brown38 suggest that an average of these may be as realistic.Since their error must therefore be about ±0.03 A, a more reliable S-O distance is probably given by the present study.The S-C distance of 1.73 A is on the short end of the range reported for S-C bond lengths39 although a bond length of 1.79 Á is reported for the case of dimethyl sulfone.40 The bond lengths and angles in the f-butyl group appear normal.The azetidinium ring geometry is similar to that of azetidinecarboxylic acid:4 the ring is not planar, the bond angles are remarkably close to 90°, and the bond lengths are almost the same.The C-N distances, 1.525 A, are not significantly different from the C-C ring distances of 1.528 A, although these are not so close in azetidinecarboxylic acid. The average ring distance in