Acid-base properties of purine residues and the effect of metal ions: Quantification of rare nucleobase tautomers*

The macro acidity constants valid for aqueous solutions of several adenine, guanine, and hypoxanthine derivatives are summarized. It is shown how the application of the corresponding constants, e.g., for 7,9-dimethyladenine, allows a quantification of the intrinsic acidic properties of the (N1)H0/+ and (N7)H+ sites via micro acidity constants, and how to use this information for the calculation of the tautomeric ratios regarding the monoprotonated species, that is, N7-N1 H versus H N7-N1, meaning that in one isomer H+ is at the N1 site and in the other at N7. It is further shown that different metal ions coordinated to a given site, e.g., N7, lead to a different extent of acidification, e.g., at (N1)H; the effect decreases in the series Cu2+ > Ni2+ > Pt2+ ~ Pd2+. Moreover, the application of micro acidity constants proves 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, an apparently increased basicity of N7 upon Pt2+ coordination at (N1)–/0 sites disappears if the micro acidity constants of the appropriate isocharged tautomers of the ligand are properly taken into account. There is also evidence that proton–proton interactions are more pronounced than divalent metal ion–proton interactions, and that these in turn are possibly larger than divalent metal ion–metal ion interactions. The indicated quantifications of the acid-base properties are meaningful for nucleic acids including the formation of certain nucleobase tautomers in low concentrations, which could give rise to mutations.