STEADY-STATE KINETICS, MICELLAR EFFECTS, AND THE MECHANISM OF PEROXIDASE-CATALYZED OXIDATION OF N-ALKYLFERROCENES BY HYDROGEN-PEROXIDE

Kinetics of the steady-state oxidation of n-alkylferrocenes (alkyl = H , Me, Et, Bu and C-5-H-11) by H2O2 to form the corresponding ferriceni um cations catalyzed by horseradish peroxidase has been studied in mic ellar systems of Triton X-100, CTAB, and SDS, mostly at pH 6.0 and 25 degrees C. The rate of oxidation of ferrocenes with longer alkyl radic als is too slow to be measured. The reaction obeying the [RFc]:[H2O2] = 2:1 stoichiometry is strictly first-order in both HRP and RFc in a w ide concentration range. The corresponding observed second-order rate constants k, which refer to the interaction of the peroxidase compound II (HRP-II) with RFc, decrease with the elongation of the alkyl subst ituent R, and this in turn is accompanied by an increase in the formal redox potentials E degrees' in the same medium. Increasing the surfac tant concentration lowers the rate constants k, the effect being due t o the nonproductive binding of RFc to micelles rather than to enzyme i nactivation. The micellar effects are accounted for in terms of the Be rezin pseudo-phase model of micellar catalysis applied to the interact ion of enzyme with organometallic substrates. The oxidation was found to occur primarily in the aqueous pseudo-phase and the calculated intr insic second-order rate constants k, are (1.9 +/- 0.5)x10(5), (2.7 +/- 0.1)x10(4), and (5.9 +/- 0.6)x10(3) M-1 s(-1) for HFc, EtFc, and n-Bu Fc, respectively. The data obtained were used for estimating the self- exchange rate constants for the HRP-II/HRP couple in terms of the Marc us formalism.