Conformations of the nucleoside analogs formycin, 2-azaadenosine, and nebularine in solution

The ribose conformations of formycine, 2-azaadenosine, nebularine, 8-bromoinosine, and 8-bromoxanthosine have been studied using proton magnetic resonance in ND3 solutions, in D2O solutions, and in pyridine solutions. The temperature was varied between -60 degrees C and +40 degrees C in ND3 and between +10 degrees C and +60 degrees C in D2O solutions. In solution, 2-azaadenosine and neubularine have a conformational behaviour similar to that of the common purine (beta) ribosides. This is in agreement with the conformations observed in the solid state. The conformations of formycin and formycin B have strong analogies with those of the 8-bromopurineribosides and differ significantly from those of the 8-azapurineribosides since they adopt preferentially the syn-S-g+ conformation. This conformation is very probably stabilized by an intramolecular hydrogen bond between between O(5 degrees) and N (3).