A glance at the reactivity of osma(II)cycles [Os(C-N)x(bpy)3-x](m+) (x=0-3) Covering a 1.8 V potential range toward peroxidase through Monte Carlo Simulations ((-)C-N=o-2-phenylpyridinato, bpy=2,2'-bipyridine).

Three cyclometalated and one coordination compounds [Os(C-N)x(bpy)3-x](m) (x/m=0/2+ (4); 1/1+ (3); 2/1+ (2); 3/0 (1); (-)C-N=2-phenylpyridinato, bpy=2,2'-bipyridine) with drastically different reduction potentials have been used for analyzing the second-order rate constants for one-electron, metal-based osmium(II) to osmium(III) oxidation of the complexes by compound I (k2) and compound II (k3) of horseradish peroxidase. Previously unknown k2 and k3 have been determined by digital simulation of cyclic voltammograms measured in phosphate buffer of pH7.6 and 21 ± 1°C. Osmium(II) species derived from osmium(III) complexes 1 and 2 were generated electrochemically in situ. Under the conditions used the reduction potentials for the Os(III/II) feature equal -0.90, -0.095, 0.23 and 0.85V versus NHE (normal hydrogen electrode) for 1-4, respectively. The rate constants k2 equal ~5 × 10(7), 6 × 10(8), 2 × 10(6) and 1 × 10(5)M(-1)s(-1) and the rate constants k3 equal ~9 × 10(6), 4× 10(7), 1 ×10(6) and 1 × 10(5)M(-1)s(-1) for complexes 1-4, respectively. Both rate constants k2 and k3 first increase with increasing the reaction driving force on going from 4 to 2 but then both decrease on going to complex 1 though the reaction driving force is the highest in this case. The system described has been explored theoretically using docking Monte Carlo simulations.

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-[RuCl(2)(LL)(2)], [RuCO(3)(LL)(2)], cis-[RuCO(3)-(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-C(6)H(4)-2-py)(phen)(2)]PF(6) (1), [Ru(o-C(6)H(3)-p-R-2-py)(bpy)(MeCN)(2)]PF(6) (2), and [Ru(o-C(6)H(3)-p-R-2-py)(phen)(MeCN)(2)]PF(6) (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 degrees 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 x 10(4) M(-1) s(-1) at pH 4.5 and 25 degrees 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) (k(1)); E(red) + 1/4O(2) --> E(ox) (k(2)), with the rate constants k(1) in the range (2-9) x 10(7) M(-1) s(-1) under the same conditions. The huge values of k(1) 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 k(2), that is, the rate constant for reoxidation for laccases by dioxygen. The mechanism proposed was confirmed by the direct stopped-flow measurements of the k(2) rate constant (8.1 x 10(5) M(-1) s(-1) at 26 degrees C) and supported by the theoretical modeling of interaction between the bpy analogue of 1 and Coriolus hirsutes laccase using Monte Carlo simulations.

Dynamic docking and electron-transfer between cytochrome b5 and a suite of myoglobin surface-charge mutants. Introduction of a functional-docking algorithm for protein-protein complexes.

Horse myoglobin (Mb) provides a convenient "workbench" for probing the effects of electrostatics on binding and reactivity in the dynamic [Mb, cytochrome b(5)] electron-transfer (ET) complex. We have combined mutagenesis and heme neutralization to prepare a suite of six Mb surface-charge variants: the [S92D]Mb and [V67R]Mb mutants introduce additional charges on the "front" face, and incorporation of the heme di-ester into each of these neutralizes the charge on the heme propionates which further increases the positive charge on the "front" face. For this set of mutants, the nominal charge of Mb changes by -1 to +3 units relative to that for native Mb. For each member of this set, we have measured the bimolecular quenching rate constant (k(2)) for the photoinitiated (3)ZnDMb --> Fe(3+)b(5) ET reaction as a function of ionic strength. We find: (i) a dramatic decoupling of binding and reactivity, in which k(2) varies approximately 10(3)-fold within the suite of Mbs without a significant change in binding affinity; (ii) the ET reaction occurs within the "thermodynamic" or "rapid exchange" limit of the "Dynamic Docking" model, in which a large ensemble of weakly bound protein-protein configurations contribute to binding, but only a few are reactive, as shown by the fact that the zero-ionic-strength bimolecular rate constant varies exponentially with the net charge on Mb; (iii) Brownian dynamic docking profiles allow us to visualize the microscopic basis of dynamic docking. To describe these results we present a new theoretical approach which mathematically combines PATHWAY donor/acceptor coupling calculations with Poisson-Boltzmann-based electrostatics estimates of the docking energetics in a Monte Carlo (MC) sampling framework that is thus specially tailored to the intermolecular ET problem. This procedure is extremely efficient because it targets only the functionally active complex geometries by introducing a "reactivity filter" into the computations themselves, rather than as a subsequent step. This efficiency allows us to employ more computationally expensive and accurate methods to describe the relevant intermolecular interaction energies and the protein-mediated donor/acceptor coupling interactions. It is employed here to compute the changes in the bimolecular rate constant for ET between Mb and cyt b(5) upon variations in the myoglobin surface charge, pH, and ionic strength.