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)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
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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