M. Cusumano et al., Catalysis and inhibition of ligand substitution in palladium(II) square-planar complexes: Effects of DNA, J AM CHEM S, 123(9), 2001, pp. 1914-1919
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.