Electron-transfer (ET) theory describes rates in terms of nuclear-reorganization ( ) and electronic-coupling (//AB) parameters.1 These parameters are most directly determined from the driving-force dependence of the ET rate (ideally at high driving forces in the neighborhood of X).2 Remarkably slow ET rates have been observed at low driving forces (-(7°< 0.3 eV) in certain iron-sulfur3 and blue copper proteins,4 and at high driving forces in Ru(bpy)2L(His-33) (bpy = 2,2'-bipyridine; L = imidazole, pyridine, H20; His = histidine) derivatives of cytochrome c (cyt c).5 Since the latter results conflict sharply with the much faster ET rates reported for Ru-modified Zn-substituted cytochrome c (Ru-Zn-cyt c)2,6 and Ru(bpy)2(dcbpy)-labeled ferrocytochrome c (dcbpy = dicarboxybipyridine),7,8 we have determined the Ru(bpy)2L(His-33)-cyt c kinetics by using a novel flash-quench method that allows the observation of rates over an extremely wide range.9"11The rate of intramolecular oxidation of horse heart ferrocytochrome c by Ru(bpy)2(im)(His-33)3+ (im = imidazole) 12,13
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