CRYSTAL AND MOLECULAR-STRUCTURES OF 2 ADENOSINE RECEPTOR LIGANDS - 1,3-DIMETHYLXANTHINE-7-RIBOSIDE MONOHYDRATE (DMX-7R), A PARTIAL A(1) AGONIST, AND -2-(FURYL)[1,2,4]TRIAZOLO[1,5-C]QUINAZOLIN-5-AMINE METHANE SULFONATE (CGS-15943), A SELECTIVE A(2A) ANTAGONIST

The structures of two adenosine receptor ligands were determined from single-crystal X-ray diffraction analysis: DMX-7R, 1,3-dimethylxanthin e-7-riboside monohydrate, monoclinic P2(1),, a = 4.633(2), b = 6.755(1 ), c = 22.921(3) Angstrom, beta = 92.11(3)degrees, final R = 0.034 for 1441 observed reflections; CGS 15943, -(2-furyl)[1,2,4]triazolo[1.5-c ]quinazolin-5-amine methane sulfonate, monoclinic, P2(1)/n, a = 7.887( 3), b = 10.014(2), c = 20.601(5) Angstrom, beta 95.36(3)degrees, final R = 0.059 for 2150 observed reflections. DMX-7R, a partial A(1) agoni st, is a nucleoside which displays an anti conformation around the gly cosidic bond and a T-3(2) conformation of the ribose ring. Extensive h ydrogen bonds are observed which include all the hydroxyl groups of th e ribose, a water molecule and the sp(2) hybridized N9 atom. CGS 15943 is a selective A(2a) antagonist. Its structure presents some analogie s with that of adenine. The lack of the ribose ring is a necessary con dition for antagonistic activity, while the furane substituent and the fused chlorophenyl ring act as modulators of the A(2) selectivity. Th e molecule, roughly planar, is protonated at the imino nitrogen of the pyrimidine ring and forms strong hydrogen bonds with two oxygens of t he methane sulfonate ion.