Rates of Heme Oxidation and Reduction in Ru(His33)cytochrome c at Very High Driving Forces

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Abstract

The rates of Ru(His33)cytochrome c electron-transfer (ET) reactions have been measured over a driving-force range of 0.59 to 1.89 eV. The driving-force dependence of Fe2+ → Ru3+ ET in RuL2(im)(His33)cyt c [L = 2,2‘-bipyridine (bpy), 4,4‘,5,5‘-tetramethyl-2,2‘-bipyridine (4,4‘,5,5‘-(CH3)4-bpy), 4,4‘-dimethyl-2,2‘-bipyridine (4,4‘-(CH3)2-bpy), 4,4‘-bis(N-ethylcarbamoyl)-2,2‘-bipyridine (4,4‘-(CONH(C2H5))2-bpy), 1,10-phenanthroline (phen); im = imidazole] is well described by semiclassical ET theory with kmax = 2.7 × 106 s-1 (HAB = 0.095 cm-1) and λ = 0.74 eV. As predicted by theory, the rate of an exergonic (−ΔG° = 1.3 eV) heme reduction reaction, *Ru2+(bpy)2(im)(His) → Fe3+, falls in the inverted region (k = 2.0 × 105 s-1). In contrast, the rates of three highly exergonic heme reductions, *Ru2+(phen)2(CN)(His) → Fe3+ (2.0 × 105 s-1; 1.40 eV), Ru+(4,4‘-(CONH(C2H5))2-bpy)2(im)(His) → Fe3+ (2.3 × 105 s-1; 1.44 eV), and Ru+(phen)2(CN)(His) → Fe3+ (4.5 × 105 s-1; 1.89 eV), are much higher than expected for reactions directly to ground-state products. Agreement with theory is greatly improved by assuming that an electronically excited ferroheme (Fe2+ → *Fe2+; ∼ 1.05 eV) is the initial product in each of these reactions.

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