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This study is dedicated to Professor Dr. R. Bruce Martin, University of Virginia, Charlottesville (USA), on the occasion of his 70th birthday, with the very best wishes of the authors for all his future endeavors and with deep appreciation for friendship and unselfish advice provided over many years to H.S. and B.L.Abstract: The effect of Ni 2 , Cu 2 , and cis-a 2 Pt 2 or trans-a 2 Pt 2 (where a NH 3 or CH 3 NH 2 ), if coordinated to the N7 site of guanine residues, on the acid ± base properties of complexes containing guanine derivatives as ligands is considered. The various acidity constants were determined by potentiometric pH titrations. Over 60 acidity constants are listed; about half of these are new. In many instances micro acidity constants have been derived that allow a quantification of the intrinsic acid ± base properties of a certain site, which are otherwise blurred by the pK a values of overlapping buffer regions. This material allows many comparisons; among these is the observation that the acidifying properties of (N7)-coordinated divalent metal ions on the corresponding (N1)H sites in a guanine derivative decrease in the following series:The data also indicate that the effects are similar for guanine and hypoxanthine residues, but that they are more pronounced for adenine derivatives because in the latter case a (N7)-bound M 2 affects a (N1)H site; hence, a further charge effect is operative here. The available material does not yet allow certain prediction of the more subtle differences occurring between the cis and trans isomers of Pt 2 complexes, but replacement of, for example, NH 3 in the coordination sphere of Pt 2 with CH 3 NH 2 has an effect. Of course, as one might expect, the effect of (N7)-bound Pt 2 in guanine nucleotide complexes is smaller on the more remote phosphate groups than it is on the closer (N1)H sites. By evaluation (by means of micro acidity constants) of data available for hypoxanthine derivatives it is also shown that (N1) À -bound Pt 2 has an acidifying effect on the (N7)H site comparable to that of (N7)-coordinated Pt 2 on the (N1)H site.

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Crystal structure studies indicate that metalation of the exocyclic amino group of cytosine and adenine nucleobases by PtII and HgII entities, respectively, induces protonation of a nucleobase ring nitrogen atom, and hence, causes a proton shift from an exocyclic to an endocyclic N atom. This metal-assisted process thus leads to the generation of rare nucleobase tautomers. In principle, such processes can lead to the stabilization of mispairs. The present study reports the first quantum chemical analysis of the metal-assisted tautomerization. The calculations clearly demonstrate that metalation of the exocyclic amino group of nucleobases significantly increases the protonation energy of the aromatic rings of nucleobases by about 30−34 kcal/mol for the PtII adduct and by about 10−14 kcal/mol for the HgII adduct. The calculations suggest that this kind of metalation could, besides the structural changes of DNA, significantly enhance the probability of formation of mispairs in DNA. In the course of the study, we have realized a substantial difference in terminology, which is used in computational chemistry and in bioinorganic chemistry to characterize the tautomerism of nucleobases. The difference arises since nucleobases are studied in very different environments by quantum chemical and experimental bioinorganic methods. This point is clarified and discussed in detail because it is essential for future studies of metal-assisted tautomerism of nucleobases.

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