Intramolecular electron-transfer reactions of cytochrome b5 covalently bonded to ruthenium(II) polypyridine complexes: Reorganizational energy and pressure effects

Jill R. Scott, Jill L. Fairris, Mark McLean, Kefei Wang, Stephen G. Sligar, Bill Durham, Francis Millett

Research output: Contribution to journalArticle

Abstract

A series of ruthenium polypyridine complexes have been covalently attached to Cys73 of the T73C variant of synthetic rat liver cytochrome b5. Photoexcitation of the ruthenium complex results in transient electron transfer from the excited ruthenium complex, Ru(II)*, to the Fe(III) center of the protein followed by a rapid electron-transfer reaction from Fe(II) to Ru(III). The observed rate constants for the excited-state reaction ranged from 3.4 × 107 to 5 × 104 s-1 and from 4.3 × 107 to 4.5 × 105 s-1 for the back reaction. The free energy dependence of the rate constants is best described by a reorganizational energy = 1 eV and an electron tunneling matrix element HAB = 0.35 cm-1 for electron transfer between the heme iron and the ruthenium complex. The reorganizational energy is the same as that obtained with the T65C variant labeled at Cys65 with the same series of ruthenium complexes. HAB, however, is 30% larger in the T73C derivatives. The pressure dependence of the rate constants of two derivatives of cytochrome b5 have been determined in order to investigate the sensitivity of the electronic coupling to pressure dependent van der Waals contacts between the redox centers. The first derivative was made by labeling the T65C variant with Ru((CH3)2bpy)2(CH3bpyCH 2-)2+ and contains a well defined 12 bond covalent link between the redox centers. The T65C derivative shows no pressure dependence over the range of 0.1-200 MPa. The second derivative was made by labeling the T73C variant with the same complex. In this case, the electronic coupling between the redox centers appears to involve a van der Waals contact between the hydrogens on the γ-carbon of Ser71 and a porphyrin methyl group. This derivative shows a very small negative volume of activation.

Original languageEnglish (US)
Pages (from-to)193-200
Number of pages8
JournalInorganica Chimica Acta
Volume243
Issue number1-2
DOIs
StatePublished - Feb 29 1996

Keywords

  • Cytochrome b complexes
  • Electron transfer
  • Polypyridine complexes
  • Ruthenium complexes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry
  • Materials Chemistry

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