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Title: Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides

Abstract

The mechanism of the proton-coupled electron transfer reaction, Q{sub A}{sup -}Q{sub B}{sup -} + H{sup +} {yields} Q{sub A}(Q{sub B}H){sup -} (i.e. k{sup (2)}{sub AB}), was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the Q{sub A} site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the Q{sub B} site. The measured rate constants, k{sup (2)}{sub AB}, increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy). The proton-coupled electron transfer was modeled. The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The best agreement with the experimental data is given by a two-step mechanism in which fast reversible proton transfer is followed by rate limiting electron transfer. For this mechanism the observed free energy dependence for k{sup (2)}{sub AB} can be fitted using reasonable parameters of the Marcus theory. The free energy dependence predicted using a simple model for a concerted reaction also provides a reasonable fit to the data. 75 refs., 9 figs., 2 tabs.

Authors:
; ; ;  [1];  [2]
  1. Univ. of California, San Diego, CA (United States)
  2. Univ. of Manchester (United Kingdom)
Publication Date:
OSTI Identifier:
420820
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the American Chemical Society; Journal Volume: 118; Journal Issue: 38; Other Information: PBD: 25 Sep 1996
Country of Publication:
United States
Language:
English
Subject:
40 CHEMISTRY; 55 BIOLOGY AND MEDICINE, BASIC STUDIES; 99 MATHEMATICS, COMPUTERS, INFORMATION SCIENCE, MANAGEMENT, LAW, MISCELLANEOUS; QUINONES; REDUCTION; PHOTOCHEMICAL REACTIONS; BACTERIA; FREE ENERGY; CHEMICAL REACTION KINETICS; MATHEMATICAL MODELS; PHOTOSYNTHESIS

Citation Formats

Graige, M.S., Paddock, M.L, Feher, G., Okamura, M.Y., and Bruce, J.M. Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides. United States: N. p., 1996. Web. doi:10.1021/ja960056m.
Graige, M.S., Paddock, M.L, Feher, G., Okamura, M.Y., & Bruce, J.M. Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides. United States. doi:10.1021/ja960056m.
Graige, M.S., Paddock, M.L, Feher, G., Okamura, M.Y., and Bruce, J.M. Wed . "Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides". United States. doi:10.1021/ja960056m.
@article{osti_420820,
title = {Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides},
author = {Graige, M.S. and Paddock, M.L and Feher, G. and Okamura, M.Y. and Bruce, J.M.},
abstractNote = {The mechanism of the proton-coupled electron transfer reaction, Q{sub A}{sup -}Q{sub B}{sup -} + H{sup +} {yields} Q{sub A}(Q{sub B}H){sup -} (i.e. k{sup (2)}{sub AB}), was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the Q{sub A} site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the Q{sub B} site. The measured rate constants, k{sup (2)}{sub AB}, increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy). The proton-coupled electron transfer was modeled. The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The best agreement with the experimental data is given by a two-step mechanism in which fast reversible proton transfer is followed by rate limiting electron transfer. For this mechanism the observed free energy dependence for k{sup (2)}{sub AB} can be fitted using reasonable parameters of the Marcus theory. The free energy dependence predicted using a simple model for a concerted reaction also provides a reasonable fit to the data. 75 refs., 9 figs., 2 tabs.},
doi = {10.1021/ja960056m},
journal = {Journal of the American Chemical Society},
number = 38,
volume = 118,
place = {United States},
year = {Wed Sep 25 00:00:00 EDT 1996},
month = {Wed Sep 25 00:00:00 EDT 1996}
}