Abstract
Abstract The crystal structure of 5-methylcytosine hemihydrate has been determined by an X-ray analysis to investigate the substitution effect on the cytosine moiety. The space group is C2/c, with unit-cell dimensions a=17.832(4), b=8.261(2), c=10.641(3) Å, β=125.12(1)°, and Z=8. The hydrogen bonding scheme is quite different from that of cytosine, its monohydrate, 5-bromocytosine, and its dioxane hemi-solvate. This is ascribed to the packing effect of the methyl group. A comparison with the cytosine structure indicates some deviations in bond lengths and angles, which are due to the steric and electronic effects caused by methylation. The similar values of the C(2)–N(3)–C(4) angle in cytosine and 5-methylcytosine can be related to a resemblance of their pKa values.
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