Comparative Kinetic Analysis of Reversible Intermolecular Electron-Transfer Reactions between a Series of Pentaammineruthenium Complexes and Cytochrome c Martin Meier, † Ji Sun, ‡ James F. Wishart,* ,‡ and Rudi van Eldik* ,† Institute for Inorganic Chemistry, University of Erlangen-Nu ¨rnberg, Egerlandstrasse 1, 91058 Erlangen, Germany, and Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 ReceiVed NoVember 23, 1994 X 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. Introduction Electron transfer plays an important role in biological processes such as respiration and photosynthesis. Redox reactions between pairs of donors and acceptors can occur over long distances (g10 Å) in biological systems. A good example is the redox protein cytochrome c. It is a relatively small protein with a MW of ca. 12 400 which undergoes a reversible Fe(II)/ Fe(III) redox reaction. Electron-transfer reactions of cytochrome c have been widely studied and remain subjects of continued interest. For example, intra- and intermolecular electron transfer studies have been performed using pulse radiolysis, flash photolysis, or stopped-flow techniques on cytochrome c and redox center-modified cytochrome c. 1-6 Previous kinetic studies have shown that cytochrome c is oxidized by a large number of redox complexes such as Co(phen) 3 3+ and Ru(NH 3 ) 5 py 3+ (py ) pyridine) via outer-sphere mechanisms. 7,8 The reaction site is expected to be in the vicinity of the partially exposed heme edge. It has been proposed 8 that the π-conjugated pyridine ligand in the latter system is able to penetrate into the interior of the protein, whereas Ru(NH 3 ) 6 3+ is not able to penetrate into the protein surface. Furthermore, it was proposed that the access of the complex to this heme edge depends on the properties of the reactants, i.e. size, charge, and surface properties. 8 In this work, the substituents on the pyridine ring were varied in order to determine if interactions between the amino acid side chain on the protein and the pyridine ring can affect the reaction. If the substituted pyridine ring is not able to penetrate completely into the pocket close to the heme edge, the reaction rate will probably be unusually low due to the increase in distance between the redox centers. This study includes a detailed kinetic and thermodynamic analysis of the electron-transfer reactions between cytochrome c and several pentaammineruthenium complexes. Due to the low driving force of these systems, we were able to follow the reactions in both directions. The combination of activation volumes for the forward and reverse reactions, together with the overall reaction volume determined for these reactions, enabled us to construct volume profiles for the overall processes. In a previous study 9 we showed that the transition state for the Ru(NH 3 ) 5 (isonicotinamide) 2+/3+ /cytochrome c system lies half- way between the reactant and product states on a volume basis. This is in agreement with theoretical predictions based on the Marcus theory. The main volume changes were assumed to arise from electrostriction effects on the metal complex, since cytochrome c shows only a very small volume change during the redox process. 10 Modifications of the ligand on the ruthenium ammine complexes may affect the penetration in the precursor complex as outlined above and so influence the position of the transition state in terms of “early” or “late” along the reaction coordinate for the electron-transfer process, which should clearly show up in the volume profile. Experimental Section Materials. Horse heart cytochrome c (type VI, Sigma) was purified and reduced as reported previously. 9 The concentrations of the cytochrome solutions were determined by UV/vis spectroscopy. All * To whom correspondence should be addressed. † University of Erlangen-Nu ¨rnberg. ‡ Brookhaven National Laboratory. X Abstract published in AdVance ACS Abstracts, February 15, 1996. (1) Therien, M. J.; Chang, I.-J.; Raphael, A. L.; Bowler, B. E. ; Gray, H. B.; Structure and Bonding; Springer Verlag: Berlin, 1991; Vol. 75, p 109. (2) Isied S. S.; Ogawa, M. Y.; Wishart, J. F. Chem. ReV. 1992, 92, 381. (3) Winkler, J. R.; Gray, H. B. Chem. ReV. 1992, 92, 369. (4) Wishart, J. F.; van Eldik, R.; Sun, J.; Su, C.; Isied, S. S. Inorg. Chem. 1992, 31, 3986. (5) Wherland S.; Gray, H. B. Biological Aspects of Inorganic Chemistry; John Wiley: New York, 1977; p 289. (6) Moore, G. R.; Eley, E. G. S.; Williams, G. In AdVances in Inorganic and Bioinorganic Mechanisms; Sykes, A. G., Ed.; Academic Press: London, 1984; Vol. 3, p 1. (7) McArdle, J. V.; Gray, H. B.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1974, 96, 5737. (8) Cummins, D.; Gray, H. B. J. Am. Chem. Soc. 1977, 99, 5158. (9) Ba ¨nsch, B.; Meier, M.; Martinez, P.; van Eldik, R.; Chang, S.; Sun, J.; Isied, S. S.; Wishart, J. F. Inorg. Chem. 1994, 33, 4744. (10) Sun, J.; Wishart, J. F.; van Eldik, R.; Shalders, R. D.; Swaddle, T. W. J. Am. Chem. Soc. 1995, 117, 2600. 1564 Inorg. Chem. 1996, 35, 1564-1570 0020-1669/96/1335-1564$12.00/0 © 1996 American Chemical Society