Noncovalent Interactions of Platinum(II) Square Planar Complexes Containing Ligands Out-of-Plane with DNA

The interaction of the complexes
[Pt(bipy)(4-Rpy)2]2+ and
[Pt(4,4‘-Ph2bipy)(4-Rpy)2]2+
(Ph = phenyl; bipy =
2,2‘-bipyridine; R = H, CN, CH3, NH2) with
DNA has been studied with a series of techniques. The
processes
give rise to (i) lengthening of rodlike DNA and unwinding of closed
circular DNA and (ii) an increase in the
DNA melting temperature comparable with that observed for known
intercalators. In addition, the reaction of
the complexes [Pt(bipy)(py)2]2+
and
[Pt(4,4‘-Ph2bipy)(py)2]2+
is inhibited by the presence of DNA. These results
have been interpreted by assuming that the substances intercalate in
spite of the presence of ligands out of plane.
The crystal structure determined for
[Pt(4,4‘-Ph2bipy)(3,5-Me2py)2]2+
by X-ray analysis shows that also one of
the phenyl rings is twisted with respect to the square plane.
Binding constants, KB, determined
spectrophotometrically at 25 °C and pH 7 using the McGhee−von Hippel approach,
increase for both series of complexes on
increasing pKa of coordinated pyridines and are
larger for those with 4,4‘-Ph2bipy. The
increasing affinity for
DNA on increasing electron density of the interacting moiety is
accounted for by assuming that London dispersion
forces play a major role in the processes.