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Pure Appl. Chem. 76(10), 1869-1886, 2004

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

H. Sigel

Department of Chemistry, Inorganic Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland

Abstract: 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)H^0/+ 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 Pt ²⁺ acidifies the (N1)H^0/+ 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.

*Plenary lecture presented at the 28th International Conference on Solution Chemistry, Debrecen, Hungary, 23-28 August 2003. Other presentations are published in this issue, pp. 1809-1919.

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