Kinetic and Theoretical Comprehension of Diverse Rate Laws and Reactivity Gaps in Coriolus hirsutus Laccase-Catalyzed Oxidation of Acido and Cyclometalated Ru^II Complexes

The reactivity of the acido Ru^II 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 Ru^II 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 Ru^III 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 Ru^II, 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) + Ru^II → E(red) + Ru^III (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.