Intramolecular Electron Transfer in Cytochrome b5 Labeled with Ruthenium(II) Polypyridine Complexes: Rate Measurements in the Marcus Inverted Region

Jill R. Scott, Anne Willie, Mark McLean, Patrick S. Stayton, Stephen G. Sligar, Bill Durham, Francis Millett

Research output: Contribution to journalArticlepeer-review


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 × 107, 1.7 × 107, and 6 × 105 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 × 106, 5.4 × 106, and 2.3 × 106 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 β = 1.4 Å and a through space separation of 12 Å. The rate constants were independent of temperature over the range +25 to −90 °C.

Original languageEnglish (US)
Pages (from-to)6820-6824
Number of pages5
JournalJournal of the American Chemical Society
Issue number15
StatePublished - Jul 1 1993
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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