Conformation of exocyclic amino groups in purines and pyrimidines: crystal structure and conformation of 1-methyl-N4-hydroxycytosine hydrochloride

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Abstract
The hydrochloride salt of 1-methyl-N4-hydroxycytosine crystallizes in the triclinic space group P.hivin.1 with cell dimensions: a = 8.232 (1), b = 9.293 (1), c = 5.416 (1) .ANG. (1 .ANG. = 0.1 nm); .alpha. = 91.95 (1), .beta. = 91.72 (2), .gamma. = 71.56 (1).degree. (SE in parentheses). The structure was solved by direct methods and refined to R = 3.7% for 1514 reflections. Despite the absence of an intramolecular H-bond in the solid state, the N4-hydroxy substituent is syn to the ring N(3). This conformation, which probably also prevails in the neutral form, is of relevance to the mechanism of attack of cytosine by hydroxylamine, known to involve predominantly the cytosine cation, and to the mechanism of hydroxylamine mutagenesis. Such conformational aspects are also relevant to other phenomena, including translation and restriction.