Cyclometalated ruthenium(II) complexes as efficient redox mediators in peroxidase catalysis

Cyclometalated ruthenium(II) complexes, [Ru-II(Csimilar toN)(Nsimilar toN)(2)]PF6 [HCsimilar toN=2-phenylpyridine (Hphpy) or 2-(4'-tolyl)pyridine; Nsimilar toN=2,2'-bipyridine, 1,10-phenanthroline, or 4,4'-dimethyl-2,2'-bipyridine], are rapidly oxidized by H2O2 catalyzed by plant peroxidases to the corresponding Ru-III species. The commercial isoenzyme C of horseradish peroxidase (HRP-C) and two recently purified peroxidases from sweet potato (SPP) and royal palm tree (RPTP) have been used. The most favorable conditions for the oxidation have been evaluated by varying the pH, buffer, and H2O2 concentrations and the apparent second-order rate constants (k(app)) have been measured. All the complexes studied are oxidized by HRP-C at similar rates and the rate constants k(app) are identical to those known for the best substrates of HRP-C (10(6)-10(7) M-1 s(-1)). Both cationic (HRP-C) and anionic (SPP and RPTP) peroxidases show similar catalytic efficiency in the oxidation of the Ru-II complexes. The mediating capacity of the complexes has been evaluated using the SPP-catalyzed co-oxidation of [Ru-II(phpy)(bpy)(2)]PF6 and catechol as a poor peroxidase substrate as an example. The rate of enzyme-catalyzed oxidation of catechol increases more than 10,000-fold in the presence of the ruthenium complex. A simple routine for calculating the rate constant k(c) for the oxidation of catechol by the Ru-III complex generated enzymatically from [Ru-II(phpy)(bpy)(2)](+) is proposed. It is based on the accepted mechanism of peroxidase catalysis and involves spectrophotometric measurements of the limiting Ru-II concentration at different concentrations of catechol. The calculated k(c) value of 0.75 M-1 s(-1) shows that the cyclometalated Ru-II complexes are efficient mediators in peroxidase catalysis.