REACTIVITY OF THE HORSERADISH-PEROXIDASE COMPOUND-I AND COMPOUND-II TOWARD ORGANOMETALLIC SUBSTRATES - A STOPPED-FLOW KINETIC-STUDY OF OXIDATION OF FERROCENES

Reactivity of horseradish peroxidase compounds I and II (HRP-I and HRP -II) toward organometallicic substrates, viz water-soluble ferrocenes RFc (R = COOH and CH2NMe2), has been studied at 25 degrees C, pH 6.0 a nd ionic strength 0.1 M. The second-order rate constants k(2) for the reaction of HRP-I with FcCOOH and FcCH(2)NMe(2) equal (1.00+/-0.04)x10 (6) and (0.27+/-0.01)x10(6) M-1 s(-1), respectively. The values of k(3 ) for the reaction of HRP-II with FcCOOH and FcCH(2)NMe(2) equal (1.1/-0.1)x10(4) and (0.25+/-0.01)x10(4) M-1 s(-1), respectively. The stea dy-state kinetic study of the HRP-catalyzed oxidation of the ferrocene s by H2O2 under the same conditions gave the second-order rate constan ts of (0.94+/-0.03)x10(4) and (0.24+/-0.06)x10(4) M-1 s(-1) for FcCOOH and FcCH(2)NMe(2), respectively, which are in a good agreement with k (3). The results reported here confirm the proposal that the rate-limi ting step of the steady-state oxidation of ferrocenes is the electron transfer from the substrate to HRP-II.