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Electron Transfer in Ruthenium-Modified Cytochromes c. σ-Tunneling Pathways through Aromatic Residues

Casimiro, Danilo R. and Richards, John H. and Winkler, Jay R. and Gray, Harry B. (1993) Electron Transfer in Ruthenium-Modified Cytochromes c. σ-Tunneling Pathways through Aromatic Residues. Journal of Physical Chemistry, 97 (50). pp. 13073-13077. ISSN 0022-3654. doi:10.1021/j100152a007.

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The rates of intramolecular electron-transfer (ET) reactions from the ferroheme to bis(2,2'-bipyridine)(imidazole)-ruthenium(II1) complexes bound to genetically engineered histidines (58 and 66) on the surface of yeast iso-1-cytochrome c (cyt c) have been measured by using a laser flashquench technique. The crystal structure of the wild-type protein indicates that the ET pathways involve aromatic side chains: Ru(His58)cyt c includes a bridging tryptophan at position 59, and Ru(His66)cyt c has a tyrosine at 67. A variant in which the bridging Tyr67 in the His66 mutant had been replaced with a phenylalanine also was examined. The Fe^2+ → Ru^(3+)ET rate constants (25 °C, pH 7.0) are as follows: 5.2(5) × l0^4 (ΔE° = 0.69(5)), Ru(His58)cyt c; 1.0(1) × 10^6 (ΔE° = 0.72(5)), Ru(His66)cyt c; and 3.1(3) × l0^6 s^-1 (ΔE° = 0.77(5) eV), Ru(His66Phe67)cyt c. The experimentally derived electronic coupling constants [H_AB(His66)= 0.014; H_AB(HiS66)= 0.060 cm-1 are in closer agreement with the lengths of a-tunneling pathways than with the direct donor-acceptor distances, and there is no indication that the u orbitals of intervening groups enhance any of these couplings. Maximum ET rates in the modified cytochromes drop by 2 orders of magnitude for every 6.3-A increase in the a-tunneling length. Analysis of the results also suggests that an internal water molecule in Ru(His66Phe67)cyt c plays a role in linking the Ru(His66) group to the heme.

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Winkler, Jay R.0000-0002-4453-9716
Gray, Harry B.0000-0002-7937-7876
Additional Information:© 1993 American Chemical Society. Received: June 3, 1993; in final form: July 22, 1993. We thank Prabha Siddarth, Rudy Marcus, David Beratan, and Jos6 Onuchic for helpful discussions; Bill Durham for a preprint of reference IC; Albert Berghuis, Terence Lo, and Gary Brayer for information about the crystal structure of reduced yeast iso- 1-cytochrome c and the Tyr67 - Phevariant; and Michael Smith for the gift of the cytochrome c expression system. The assistance of I-Jy Chang, Bruce Bowler, Thomas Sutherland, John Racs, and Keith Herman in some of the experimental work is acknowledged. This work was supported by the National Science Foundation, the National Institutes of Health, and the Arnold and Mabel Beckman Foundation (this is contribution no. 8678 from the Beckman Institute).
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Caltech Beckman Institute8678
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Official Citation:Electron transfer in ruthenium-modified cytochromes c. .sigma.-tunneling pathways through aromatic residues Danilo R. Casimiro, John H. Richards, Jay R. Winkler, and Harry B. Gray The Journal of Physical Chemistry 1993 97 (50), 13073-13077 DOI: 10.1021/j100152a007
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ID Code:57122
Deposited On:01 May 2015 18:08
Last Modified:10 Nov 2021 21:08

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