Comparative Kinetic Analysis of Reversible Intermolecular Electron-Transfer Reactions between a Series of Pentaammineruthenium Complexes and Cytochromec

Abstract

In this kinetic and thermodynamic study, the reversible outer-sphere electron-transfer reactions between a series of Ru(NH(3))(5)L(3+/2+) complexes (L = etpy, py, lut) (etpy = 4-ethylpyridine; py = pyridine; lut = 3,5-lutidine) and cytochrome c were investigated as a function of ionic strength, buffer, pH, temperature, and pressure. Due to the low driving forces of these systems, it was possible to study all the reactions in both redox directions. The observed rate constants for various L are correlated on the basis of the ability of ligands on the ruthenium complex to penetrate the heme groove on cytochrome c. The measurements as a function of pressure enabled the construction of volume profiles for all investigated systems. The activation volumes for all of these processes are very similar: between -14.9 and -17.8 cm(3) mol(-)(1) for the reduction and between +14.7 and +17.8 cm(3) mol(-)(1) for the oxidation of the protein by Ru(NH(3))(5)L(2+/)(3+), respectively. The overall reaction volume varies between 27 and 35 cm(3) mol(-)(1), from which it follows that the transition state lies exactly halfway between reactant and product states on a volume basis in all cases. There is good agreement throughout between kinetic and thermodynamic data.

High‐Pressure Probing of a Changeover in the Charge‐Transfer Mechanism for Intact Cytochrome c at Gold/Self‐Assembled Monolayer Junctions

“…[1][2][3] Application of this technique along (or in combination) with other kinetic approaches also seems to be promising for biological processes, including charge-transfer (CT) reactions. [4][5][6] However, even for small and well-characterized proteins such as cytochrome c (CytC), [7,8] an intrinsic CT mechanism in "homogeneous" systems is difficult to recognize conclusively as a consequence of the extra structural and dynamic environmental complexity introduced by the participating redox partner. [4,5,[9][10][11] The covalent attachment of "small" complex ions as the redox counterparts at different external sites of CytC [12,13] does not warrant sufficiently smooth variation of intrinsic parameters, such as an electronic coupling (correlated with the CT distance; see below), because of the highly inhomogeneous nature of the protein interior.…”

“…[4][5][6] However, even for small and well-characterized proteins such as cytochrome c (CytC), [7,8] an intrinsic CT mechanism in "homogeneous" systems is difficult to recognize conclusively as a consequence of the extra structural and dynamic environmental complexity introduced by the participating redox partner. [4,5,[9][10][11] The covalent attachment of "small" complex ions as the redox counterparts at different external sites of CytC [12,13] does not warrant sufficiently smooth variation of intrinsic parameters, such as an electronic coupling (correlated with the CT distance; see below), because of the highly inhomogeneous nature of the protein interior. In this context, artificial bioelectrochemical devices made of electrode-deposited self-assembled monolayer (SAM) films of variable composition and thickness, and CytC or other redox proteins attached or freely diffusing to the SAM terminal groups (also subject to wide variations), were proven to be systems with wellcontrolled variable parameters, and hence suitable for fundamental studies [8,[14][15][16][17][18] and some technological applications.…”

Application of High Pressure in the Elucidation of Inorganic and Bioinorganic Reaction Mechanisms

“…The volume profile shown in Figure 7.10, which uses the same system of letters for the various states as in the free energy profile, locates the transition state early in the overall volume change, a contrasting situation from the 50% position in the overall volume change, of the transition state in redox reactions of cytochrome c with octahedral Ru(III)/Ru(II) complexes. 290,291 In the latter reactions, there is no change in coordination number, whereas in the copper-ferrozine system, the transition state occurs at a point where the electron is symmetrically poised within the cytochrome c interaction with the five-coordinate Cu (II)/Cu(I) species, and the water loss causing an intrinsic change, occurs subsequently. with the N-alkyl substituent.…”

Probing Redox Reactions of Immobilized Cytochrome c Using Evanescent Wave Cavity Ring‐Down Spectroscopy in a Thin‐Layer Electrochemical Cell

“…[7][8][9][10][11] However, since the first successful demonstrations of cyclic voltammetry of horse heart cytochrome c by Eddowes and Hill [12] and Yeh and Kuwana [13] independently in 1977, the area of the direct and mediated electrochemistry of both ET proteins and complex enzymes has flourished. [2,14] Protein film voltammetry (PFV) [15] is widely accepted as a powerful tool for understanding ET between proteins and electrodes for the fabrication of devices such as biosensors, [2,16] in bioelectronics and biofuel cells, [8,9] and as a model approach for comprehending ET between redox proteins in vivo. [17] In PFV, a potential applied at an electrode drives electrons in and out of the active sites of an electroactive protein film immobilized on the electrode, allowing the evaluation of the formal potentials and the rates of ET and other kinetic and thermodynamic parameters.…”