| Abstract: | The imino-amino tautomeric equilibrium of the promutagenic adenosine analogue N6-methoxy-2',3',5'-tri-O-methyladenosine [OMe6A(Me)3], in solvents of various polarities, has been studied with the aid of 1H and 13C NMR spectroscopy. The high energy barrier (free enthalpy delta G = 80 +/- 5 kJ X mol-1) between the two tautomeric species renders possible direct observation of the independent sets of all 1H and 13C signals from each of them. The equilibrium ranges from 10% imino in CCl4 to 90% in aqueous medium. Thermodynamic parameters, including energy barriers and lifetimes, were calculated from the temperature dependence of the equilibrium. Essentially similar results prevail for the promutagenic N6-hydroxy analogue. The conformations of the sugar moieties, and of the base about the glycosidic bond, for both tautomers are similar to those for adenosine. The conformation of the exocyclic N6-OCH3 group, which determines the ability of each species to form planar associates (hydrogen-bonded base pairs), has also been evaluated. Formation of autoassociates of OMe6A(Me)3 and of heteroassociates with the potentially complementary 2',3',5'-tri-O-methyluridine and -cytidine, in chloroform solution, was also investigated. The amino form base pairs with uridine and the imino form with cytidine. Formation of a complementary base pair by a given tautomeric species was accompanied by an increase of up to 10% in the population of this species and a concomitant decrease in population of the other species.(ABSTRACT TRUNCATED AT 250 WORDS) |
