Proton magnetic resonance studies of 2′-, 3′-, and 5′-deoxyadenosine conformations in solution

Proton magnetic resonance studies of 2′-deoxyadenosine (2′-dA), 3′-deoxyadenosine (3′-dA), 5′-deoxyadenosine (5′-dA) and 8-bromo-5′-deoxyadenosine (8-Br-5′-dA) have been carried out in the temperature range between −60° and +40°C for ND3, solutions, +40° and +100°C for D20 solutions, and finally +10° and +60°C for pyridine solutions. The analysis is based on the two-state S⇄N model of the ribose moiety proposed by Altona and Sundaralingam. In all solvents,2′-dA favours slightly the S state of the ribose and the g+ conformer of the exocyclic CH2OH group. However, 3′-dA prefers strongly the N state of the ribose and the g+ conformation. Both the S and N states of the ribose are equally favoured by 5′-dA and 8-Br-5′-dA. Evidence for the existence of an intramolecular hydrogen bond between 0(5′) and N3 in purine(β)-nucle-osides is presented. It is also concluded that cordycepin (3′-dA) exists in solution mainly in the anti conformation of the base relative to the ribose.