Intrinsic acid-base properties of purine derivatives in aqueous solution and comparison of the acidifying effects of platinum(II) coordinated to N1 or N7: Acidifying effects are reciprocal and the proton "outruns" divalent metal ions

The effect of Pt2+ coordination, in particular of (dien)Pt2+ or cis-(NH3)(2)Pt2+, on the acid-base properties of the purine ligands 9-ethylguanine (9EtG), 9-methylhypoxanthine (9MeHx), inosine (Ino), 9-methyladenine (MA), and N6',N6',N9-trimethyladenine (TriMeA) is quantitatively evaluated. The corresponding acidity constants of the complexes are calculated by curve-fitting procedures using previously published H-1 NMR shift data which had been measured in aqueous solution (D2O) in dependence on pH (pD). Comparison of the pK(a) values of the ligands with those of the Pt2+ complexes reveals the expected behavior for the (N7)-platinated complexes; i.e., the (N1)H0/+ sites are acidified due to charge repulsion. However, Pt2+ coordination at (N1)(-/0) sites leads to an (already previously observed) apparent increase in the basicity of the N7 sites for the guanine, hypoxanthine, and adenine residues; this is also the case if Pt2+ is bound to N3. Coordination of Pt2+ to both the (N1)(-) and N7 sites of 9EtG results apparently in an enhanced basicity of N3 if compared with the release of the proton from the (N3)H+ site in H-2(9EtG)(2+). For the former cases in aqueous solution (H2O) it is now proven for a comprehensive set of data (seven examples), by taking into account the intrinsic basicities of the various N7 sites via micro acidity constants, that the acidifications are reciprocal and identical. This means Pt2+ coordinated to (N1)(-/0) sites in guanine, hypoxanthine, or adenine residues acidifies the (N7)H+ unit to the same extent as (N7)-coordinated Pt2+ acidifies the (N1)H0/+ site. In other words, the apparently increased basicity of N7 upon Pt2+ coordination at (Nl)-10 sites disappears if the micro acidity constants of the appropriate isocharged tautomers of the ligand are properly taken into account. It is further proven, on the basis of the evaluations of the nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), that these given conclusions are also valid for nucleotides. In addition, it is shown that the mentioned apparent basicity increase, which results from the use of macro acidity constants, has its origin in the fact that the proton-metal ion (pt2+) interaction (the extent of which depends on the kind of metal ion involved) is less pronounced than the proton-proton interaction. Finally, the proven reciprocal behavior will now allow one to determine micro acidity constants of ligands by studying complexes formed with kinetically inert metal ions. A further result of interest is the proof that the competition of Pt2+ (or Pd2+) with the proton for the (N1)(-) and N7 binding sites of inosinate results in the isomer where the metal ion is at N7 with the proton relegated to (N1)(-); this isomer is favored by a factor of about 2000 compared with the one having the metal ion at (N1)(-) and the proton at N7.