MOLECULAR-STRUCTURE OF 2-CHLORO-2'-DEOXYADENOSINE AN AGENT USED IN THERAPY FOR MULTIPLE-SCLEROSIS AND ITS UNUSUAL CRYSTAL PACKING MODE

The X-ray crystal structure of 2-chloro-2'-deoxyadenosine was determin ed at low temperature [space group orthorhombic P2(1)2(1)2, a = 7.493( 6), b = 16.894(4), c = 9.065(4) Angstrom, V = 1147.6(11) Angstrom(3), 4097 unique reflections, R = 0.050]. The ribose unit is in T-2(3) conf ormation, and the orientation around the Cl'-N glycosylic bond is syn with respect to the base. The 2-chloro-substitution leads to small but significant changes of the covalent geometry of the base at the subst itution site, particularly to widening of the endocyclic angle at C2. The crystal packing is unusual foradenine nucleosides: molecules form stacks with parallel bases which are aligned by chains of short chlori ne-chlorine contacts. (C) 1998 Elsevier Science B.V.