Kinetic and Theoretical Comprehension of Diverse Rate Laws and Reactivity Gaps in Coriolus hirsutus Laccase-Catalyzed Oxidation of Acido and Cyclometalated RuII Complexes
2009-06-02T00:00:00Z (GMT) by
The reactivity of the acido RuII complexes cis-[RuCl2(LL)2], [RuCO3(LL)2], cis-[RuCO3-(bquin)2] (LL = 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen); bquin = 2,2′-biquinoline) and cyclometalated RuII derivatives of 2-phenylpyridine and 4-(2-tolyl)pyridine [Ru(o-C6H4-2-py)(phen)2]PF6 (1), [Ru(o-C6H3-p-R-2-py)(bpy)(MeCN)2]PF6 (2), and [Ru(o-C6H3-p-R-2-py)(phen)(MeCN)2]PF6 (3) (R = H (a), Me (b)) toward laccase from Coriolus hirsutus has been investigated by conventional UV−vis spectroscopy at pH 3−7 and 25 °C. The acido and cyclometalated complexes are readily oxidized into the corresponding RuIII species, but the two types of complexes differ substantially in reactivity and obey different rate laws. The acido complexes are oxidized more slowly and the second-order kinetics, first-order in laccase and RuII, holds with the rate constants around 5 × 104 M−1 s−1 at pH 4.5 and 25 °C. The cyclometalated complexes 1−3 react much faster and the hyperbolic Michaelis−Menten kinetics holds. However, it is not due to formation of an enzyme−substrate complex but rather because of the ping-pong mechanism of catalysis, viz. E(ox) + RuII → E(red) + RuIII (k1); E(red) + 1/4O2 → E(ox) (k2), with the rate constants k1 in the range (2−9) × 107 M−1 s−1 under the same conditions. The huge values of k1 move the enzymatic oxidation toward a kinetic regime when the dioxygen half-reaction becomes the rate-limiting step. Cyclometalated compounds 1−3 can therefore be used for routine estimation of k2, that is, the rate constant for reoxidation for laccases by dioxygen. The mechanism proposed was confirmed by the direct stopped-flow measurements of the k2 rate constant (8.1 × 105 M−1 s−1 at 26 °C) and supported by the theoretical modeling of interaction between the bpy analogue of 1 and Coriolus hirsutes laccase using Monte Carlo simulations.