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 †

Abstract

The stability constants of the mixed-ligand complexes formed between Cu(arm)²⁺, where arm = 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen), and 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 °C and I = 0.1 mol dm^–3 (NaNO₃). A microconstant 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; similarly, in the ternary Cu(arm)(H;dGMP)⁺ complexes the Cu(arm)²⁺ unit is also at N7 and the proton at the phosphate residue, i.e., 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 intramolecular stack (st) formation between the aromatic ring systems of phen or bipy and the purine moiety of dGMP^2–. Macrochelate formation of a phosphate-coordinated metal ion with N7 (cl = closed/N7) is insignificant in the ternary complexes, but very pronounced in the binary Cu(dGMP) complex where it reaches a formation degree of about 93%. A quantitative analysis of the intramolecular equilibria involving the three structurally different Cu(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 macrochelated Cu(phen)(dGMP)_cl/N7 species with about 6% and the stacked Cu(phen)(dGMP)_st isomer with approximately 89%; the percentages for the Cu(bipy)(dGMP) system are similar. The relevance of these results with regard to biological systems is indicated.

References

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