Anhydrous guanine: a synchrotron study.

Very small crystals of anhydrous guanine (systematic name: 2-amino-1,7-dihydro-6H-purin-6-one), C5H5N5O, were obtained from an attempted solvothermal synthesis of a potassium complex. Data were collected at 120 K using a synchrotron radiation source. There is one essentially planar molecule in the asymmetric unit. Molecules are linked to each other by NÐH N and NÐH O hydrogen bonds to form sheets, between which there are ± stacking interactions. This crystal structure determination demonstrates conclusively that, in the absence of any solvent or other molecules, guanine exists as the amino±keto tautomer in the solid state, with H atoms attached to N1 and N7 (purine numbering), unlike its monohydrate, which has H atoms on N1 and N9.