RATES OF HEME OXIDATION AND REDUCTION IN RU(HIS33)CYTOCHROME C AT VERY HIGH DRIVING FORCES

摘要

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 RuL(2)(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 k(max) = 2.7 x 10(6) s(-1) (H-AB = 0.095 cm(-1)) and lambda = 0.74 eV. As predicted by theory, the rate of an exergonic (-Delta G degrees = 1.3 eV) heme reduction reaction, *Ru2+(bpy)(2)-(im)(His) --> Fe3+, falls in the inverted region (k = 2.0 x 10(5) s(-1)). In contrast, the rates of three highly exergonic heme reductions, *Ru2+(phen)(2)(CN)(His) --> Fe3+ (2.0 x 10(5) s(-1); 1.40 eV), Ru+ (4,4'-(CONH(C2H5))(2)-bpy)(2)(im)-(His)--> Fe3+ (2.3 x 10(5) s(-1); 1.44 eV), and Ru+(phen)(2)(CN)(His) --> Fe3+ (4.5 x 10(5) 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+; similar to 1.05 eV) is the initial product in each of these reactions.