2.1. Effects of metal coordination to acid–base properties
of purine bases
Base–base recognition and hence proper Watson–Crick
pairing crucially depend on charge densities within the heterocyclic
rings, pKa values of H donors as well as pKb values
of H acceptors, and the tautomeric structures of the two bases
[33]. From this it is evident that any chemical process affecting
the complementarity of two bases, e.g. metal coordination,
will have an influence on the H bonding behavior.
Pt
II
binding to N(7) of 9-methyladenine (9-MeA) reduces
the basicity of the N(1) position by 1.5–2.5 log units [34,35],
and hence makes this site a weaker H bonding acceptor in the
Watson–Crick pair with thymine. From a limited number of
data points it appears that there is a minor influence of the
charge carried by the metal entity, e.g. [(NH3)3Pt]2q)
[(NH3)Cl2Pt])[PtCl3]. It isto be expected that the metal
entity, while decreasing the basicity of N(1),simultaneously
increases the acidity of the exocyclic amino group, even
though this effect cannot be determined accurately due to the
high pKa of this group. The question of how these two opposing
effects influence the strength of the AT pair or whether
the pairing specificity of A is affected, remains elusive. The
problem will be solved once metal complexes of A are available
that are sufficiently soluble in aprotic media such as
CHCl3, etc., to permit the determination of association
constants.