CRYSTAL STRUCTURE OF 1,3,5-TRIMETHYL-N⁴-HYDROXYCYTOSINE, AND ITS RELEVANCE TO THE MECHANISM OF HYDROXYLAMINE MUTAGENESIS

Abstract

1,3,5-Trimethyl-N⁴-hydroxycytosine, an analogue of the promutagenic N⁴-hydroxycytosine and 5-methyl-N⁴-hydroxycytosine nucleosides, crystallizes in the monoclinic space group P 2₁/n with cell dimensions at −147°C: a = 7.1481(7), b = 9.2565(5), c = 13.3086(12) Å, β = 97.90(2)°, V = 872.24(13) ų, ρ_c = 1.426 Mg m⁻³, Z = 4, F(000) = 401.39, μ = 0.91 mm⁻¹, λ(Cu) = 1.54056 Å, 20(max) = 139.3°. The crystal structure has been solved by X-ray difraction and refined to R = 3.7 % for 1457 reflections. Notwithstandin the steric hindrance imposed by methyl groups at both N(3) and C(5), the exocyclic N⁴-OH group is located essentially in the plane of the ring, giving rise to an “overcrowded” molecule, like that of 1,5,N⁴,N⁴-tetramethylcytosine. The conformational parameters have also been compared with those of a number of related and previously reported N(1)-substituted cytosines. In the present compound the N⁴-OH rotamer is in the anti conformation relative to the ring N(3), hence similar to that of one of the rotamers in N(1)-substituted N⁴-hydroxycytosine, which permits normal Watson-Crick base pairing of the latter, relevant to the mechanism of hydroxylamine mutagenesis.