Extent of intramolecular stacking interactions in the mixed-ligand complexes formed in aqueous solution by copper(II), 2,2 '-bipyridine or 1,10-phenanthroline and 2 '-deoxyguanosine 5 '-monophosphate

The stability constants of the mixed-ligand complexes formed between Cu(arm )(2+), where arm = 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen), an d the monoanion or the dianion of 2'-deoxyguanosine 5'-monophosphoric acid [H(dGMP)(-) or dGMP(2-)] were determined by potentiometric pH titration in aqueous solution at 25 degrees C and I= 0.1 mol dm(-3) (NaNO3). A microcons tant scheme reveals that in the binary Cu(H;dGMP)(+) species the metal ion is overwhelmingly bound at N7 and the proton at the phosphate group; simila rly, in the ternary Cu(arm)(H;dGMP)(+) complexes the Cu(arm)(2+) unit is al so at N7 and the proton at the phosphate residue, it., stacking plays only a very minor role in these systems. This is different in the Cu(arm)(dGMP) complexes where the observed increased complex stability is mainly due to i ntramolecular stack (st) formation between the aromatic ring systems of phe n or bipy and the purine moiety of dGMP(2-). Macrochelate formation of a ph osphate-coordinated metal ion with N7 (cl = closed/N7) is insignificant in the ternary complexes, but very pronounced in the binary Cu(dGMP) complex w here it reaches a formation degree of about 93%. A quantitative analysis of the intramolecular equilibria involving the three structurally different C u(arm)(dGMP) species is presented and it is shown that, e.g., the 'open' Cu (phen)(dGMP)(op) isomer occurs with a formation degree of about 5%, the mac rochelated Cu(phen)(dGMP)(cl/N7) species with about 6% and the stacked Cu(p hen)(dGMP)(st) isomer with approximately 89%; the percentages for the Cu(bi py)(dGMP) system are similar. The relevance of these results with regard to biological systems is indicated.