STRUCTURES OF RIBOFLAVIN TETRAACETATE AND TETRABUTYRATE - MOLECULAR PACKING MODE OF RIBOFLAVIN TETRACARBOXYLATE AND ITS EXTENSIVE STACKING AND HYDROGEN-BONDING CHARACTERISTICS

Riboflavin tetraacetate (RTAc) acetone solvate monohydrate, 2C25H2gN4O10.C3H6O.H2O, M(r) = 1165.12, orthorhombic, P2(1)2(1)2(1), a = 61.896(8), b = 11.424 (1), c = 8.134 (i) angstrom, V = 5751.5 (12) angstrom3, Z = 4, D(m) = 1.320 (3), D(x) = 1.345 g cm-3, lambda(Cu Kalpha) = 1. 5418 angstrom, mu = 7.84 cm-1, F(000) = 2456, T = 278 K, final R = 0.046 for 5060 independent observed (F(o) > 0.0) reflections. Riboflavin tetrabutyrate (RTB), C33H44N4O10, M(r) = 656.73, triclinic, P1, a = 19.409 (2), b = 15.332 (2), c = 11.784 (1) angstrom, alpha = 95.51 (1), beta = 92.17 (1), gamma = 83.59 (1)-degrees, V = 3465.1 (6) angstrom3, Z = 4, D(m) = 1.250 (2), D(x) = 1.259 g cm-3, lambda(Cu Kalpha) = 1.5418 angstrom, mu = 7.39 cm -1, F(000) = 1400, T = 255 K, final R = 0.094 for 6342 independent observed [F(o) > 3sigma(F(o))] reflections. The crystal structures of RTAc and RTB consist of two and four crystallographically independent molecules, respectively, and are stabilized by extensive stacking (aromatic spacing of 3.3-3.5 angstrom) and NH ... O=C hydrogen-bonded dimer formation between the neighboring molecules. Each of the RTAc and RTB molecules assumes an open conformation, in order to avoid short contacts among four neighboring acetyl or butyryl groups. Although the ribityl backbones are in one of the usually observed conformations, the flexibility of the ester groups contributes to the formation of many conformers in the riboflavin derivatives. The structural characteristics inherent in the isoalloxazine ring and ribityl conformation are discussed.