Intramolecular Electron-Transfer in Cytochrome-B(5) Labeled with Ruthenium(ii) Polypyridine Complexes - Rate Measurements in the Marcus Inverted Region
by Scott, Jill R.; Willie, Anne N.; McLean, Mark; Stayton, Patrick S.; Sligar, Stephen G.; Durham, Bill; Millett, Francis Spencer
Three derivatives of recombinant T65C-cytochrome b5 were produced by labeling the single sulfhydryl group at Cys-65 with (4-(bromomethyl)-4'-methylbipyridine)bis(bipyridine)ruthenium(2+), (4-bromomethyl-4'-methylbipyridine)bis(4,4'-dimethylbipyridine)ruthenium(2+), and (4-(bromomethyl)-4'-methylbipyridine)(bipyrimidine)(bipyridine)ruthenium(2+). The ruthenium labels are linked to the heme iron through a well-defined 12-covalent-bond path and provide a range of free energies of reaction which bracket the expected reorganizational energy of 0.94 eV. The rate constants for photoinduced electron transfer from Ru(II) to the ferric heme were 1.4 x 10(7), 1.7 x 10(7), and 6 x 10(5) s-1, respectively, for the three different labeled proteins. The rate constants for the thermal back-reaction from the ferrous heme to Ru(III) were 6.0 x 10(6), 5.4 x 10(6), and 2.3 x 10(6) s-1, respectively. The rate constants show an inverse dependence on driving force, as predicted by Marcus (Marcus, R. A. J. Chem. Phys. 1957, 26, 867-871), with an electronic coupling term consistent with parameters for covalent bond coupling suggested by Beratan et al. (Beratan, D. N.; Onuchic, J. N.; Betts, J. N.; Bowler, B. E.; Gray, H. B. J. Am. Chem. Soc. 1990, 112, 7915-7920). The electronic coupling is also consistent with a simple exponential decay model with beta = 1.4 angstrom and a through space separation of 12 angstrom. The rate constants were independent of temperature over the range +25 to -90-degrees-C.
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