Catalysis and inhibition of ligand substitution in palladium(II) square-planar complexes: Effects of DNA

The kinetics of substitution, by thiourea, of ethylenediamine (en) or N,N'- dimethylethylenediamine (Me(2)en) coordinated to palladium(Ir) in the compl exes [Pd(4,4'-R(2)bpy)(en)](PF6)(2) (bpy = 2,2'-bipyridine; R = H or Me), [ Pd(en)(2)](PF6)(2) and [Pd(Me(2)en)(2)](PF6)(2) have been studied at 25 deg reesC, pH 7 and various ionic strength values, in the presence of calf thym us DNA. The rate of the reaction in water depends on ionic strength, pH, an d nucleophile concentration; at fixed pH and ionic strength the k(obsd) val ues are correlated to the square of the thiourea concentration. This rate l aw is not altered by the presence of DNA, but the rate of reaction is influ enced, depending on the nature of ancillary ligand, L-L, bound to palladium . DNA inhibits the substitution process when L-L is bpy or 4,4'-Me(2)bpy an d catalyzes the same reaction when L-L is en or Me(2)en. These opposite kin etic effects can be related to the noncovalent interactions of the various complexes with the DNA double helix. Inhibition of the reactivity of the co mplexes [Pd(4,4'-R(2)bpy)(en)](2+) is due to protection of the reaction cen ter from nucleophile attack by DNA. Acceleration of the reaction when L-L i s zn or Me(2)en is related to the dependence of the rate of reaction on pH. If, due to the higher activity of water under the electric field of phosph ate groups, hydronium ion concentration on DNA surface is higher than in th e bulk solution, the enzyme-like dependence of the rate of reaction on [DNA ] is due to progressive accumulation of the complexes around the double hel ix. Regardless of the complexes' nature, the rate constant values obtained in DNA at pH 7 correspond to values determined in water at pH 5. This pH va lue on the DNA surface, lower by about two units with respect to the bulk s olution, is in good agreement with theoretical predictions. Acceleration of ethylenediamine substitution has been observed for all of the complexes st udied in the presence of sodium polyvinylsulfonate.