Add time:09/28/2019         Source:infona.pl

Several new N 1 -substituted uncommon purine nucleosides, including doridosine (1-methyl-isoguanosine; m-iG), 1-allyl-isoguanosine (a-iG) and 1-allyl-xanthosine (a-X), have been synthesized and tested as agonists for the adenosine receptors. Some have smooth muscle relaxant or negative chronotropic activities. The X-ray crystal structure of these compounds has been determined at atomic resolution in order to understand the structure-activity relationship. The structures were solved by direct methods and refined by full-matrix least-squares refinement procedure. The crystallographic parameters are: a-iG, space group P2 1 , a=10.573 (1) Å, b=21.955 (2) Å, c=14.360 (1) Å, β=110.65 (1) o , no. of 3σ Fo's=4585, R=0.047; a-X, space group P2 1 2 1 2 1 , a=16.015 (2) Å, b=16.239 (1) Å, c=5.3723 (5) Å, no. of 3σ Fo's= 1169, R=0.031. In the a-iG crystal, there are 4 independent molecules (with different conformation) per asymmetric unit. While all 4 molecules adopt anti χ C N glycosyl torsion angle, their riboses have 3 distinct puckers (C 2 ' -exo, C 2 ' -endo and C 1 ' -exo). In contrast, the a-X structure adopts a syn χ C N glycosyl torsion angle, which is stabilized by an intramolecular hydrogen bond between the N 3 or purine base and the O 5 ' of the ribose (in C 2 ' -endo pucker). Both purine bases (a-iG and a-X) are mainly in the keto tautomer form. For the isoguanine base, the averaged N 1 C 2 bond distance (1.42 Å) is significantly longer than that (1.375 Å) of the guanine base. For the xanthine base, N 3 nitrogen has an imino proton attached which is unambiguously located in the electron density map. The surprising flexibility in the ribose ring of these N 1 -substituted uncommon purine nucleosides suggests flat the ribose moiety may not participate in the binding of nucleoside to the adenosine receptors.

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