Studies on modified nucleic acid bases: structure of 3-isobutyl-1-methylxanthine

C10H14O2, M = 222.3, monoclinic, P2/c, alpha = 4.882 (3), b = 8.715 (1), c = 25.955 (3) A, beta = 92.28 (1) degree, V = 1103.4 (9) A, Z = 4, Dm = 1.34, Dx = 1.338 g cm 3, Cu K alpha, lamba = 1.5418 A, mu = 7.62 cm 1, F(000) = 472, T = 294 k, R = 0.058 for 984 reflections I greater than 3 sigma (1). The isobutyl chain is oriented almost perpendicular to the xanthine ring (C(2)-N(3)-C(31)-C(32) +/- 99.8 (4)degree]. The isobutyl chain torsion angles are N(3)-C(31)-C(32)-C(33) +/- 62.2 (4) and N(3)-C(31) C(32)-C(34) +/- 174.1 (3) degree. The structure forms self-paired dimers of xanthine bases with a pair of N-H...O and a pair of weaker C-H...N hydrogen bonds across centers of inversion. There is a partial stacking of the xanthine bases.