The kinetics of substitution of pyridine or 2-methylpyridine, by iodide or
thiourea, in the complexes [Pt(4'-R'terpy)(2-Rpy)](BF4)(2) (R' = o-tolyl or
H; R = H or CH3) has been studied, at 25 degrees C, pH 7, and various ioni
c strength values, in the presence of and without calf thymus DNA. The reac
tions occur in one observable step, and plots of k(obsd) against nucleophil
e concentration give straight lines with zero intercepts. DNA inhibits all
the reactions studied without altering the rate law; the second-order rate
constants k(2) decrease systematically on increasing DNA concentration and
are larger at higher ionic strength values. Partitioning of the ionic react
ants in solution on electrostatic grounds can account for this kinetic effe
ct in the reaction with iodide. Iodide is kept off the double helix proximi
ty while the dicationic complexes concentrate on it. The inhibiting effect
observed for the uncharged reagent thiourea can be related to the specific
binding mode of the complexes to DNA. The complexes studied are effective i
ntercalators to double helix, and this type of interaction, which prevents
attack of thiourea at platinum, decreases their actual concentration in sol
ution. The inhibiting effect is larger for [Pt(terpy)(py)](2+) that is a be
tter intercalator. Likewise, the decrease in the rate of substitution of 2-
Rpy, at a given [DNA] on decreasing ionic strength, is due to the influence
of ionic strength on the complex-DNA interactions.