Acid-base properties of purine residues and the effect of metal ions: Quantification of rare nucleobase tautomers
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)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
Pt 2+ 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
*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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