Interactions of Cadmium II with Nucleobase Residues

The dominance of the N7 site for the coordination chemistry of the purine nucleosides and related systems is even true under rather exceptional circumstances, that 

Micro acidity constant, P hj 7 -ni for the (N7)H site of Ado under conditions where N1 does not carry a proton [36], 

Considering that the p/sTa values for both, the N7 and the N1 sites of monoprotonated adenosine are such that no competition between the proton and metal ions for binding at either site in the neutral pH range occurs, one expects that metal ions including Cd actually may coordinate at either site. Indeed, for the Cd(Ado) complex a dichotomy exists and it was previously concluded that the N7 isomer occurs with 61 10% [77] a more recent evaluation [73] arrives at 35 11%. The discrepancy between these results is no surprise as the evaluation methods differ significantly in any case, the dichotomy is certain and occurs also, e.g., with Cu(9-methyladenine) where 59 12% exist as the N7 isomer [61]. 

Evidently, equilibria (2a) and (7a) to (9a) are connected with each other via the equilibrium scheme (10) [79]  

This scheme involves four equilibrium constants and because it is of a cyclic nature, only three constants are independent of each other the fourth constant is automatically determined by the other three as follows from eq. (11)  

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