Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochrome bc1 Complex
Current competing models for the two-electron oxidation of quinol (QH{sub 2}) at the cytochrome bc{sub 1} complex and related complexes have different requirements for the reaction intermediate. At present, the intermediate species of the enzymatic oxidation process have not been observed or characterized, probably due to their transient nature. Here, we use a biomimetic oxidant, Ru(bpy){sub 2}(pbim)(PF6)2 (bpy = 2,2'-dipyridyl, pbim = 2-(2-benzimidazolate)pyridine) in an aprotic medium to probe the oxidation of the ubiquinol analogue, 2,3-dimethoxy-5-methyl-1,4-benzoquinol (UQH{sub 2}-0), an the plastoquinol analogue, trimethyl-1,4-benzoquinol (TMQH{sub 2}-0), using time-resolved and steady state spectroscopic techniques. This system qualitatively reproduces key features observed during ubiquinol oxidation by the mitochondrial cytochrome bc1 complex. Comparison of isotope dependent activation properties in the native and synthetic systems, as well as, analysis of the time-resolved direct-detection electron para magnetic resonance signals in the synthetic system allows us to conclude that: (1) the initial and rate-limiting step in quinol oxidation, both in the biological and biomimetic systems, involves electron and proton transfer, probably via a proton coupled electron transfer mechanism; (2) a neutral semiquinone intermediate is formed in the biomimetic system; and (3) oxidation of the QH*/QH{sub 2} couple for UQH{sub 2}-0, but not TMQH{sub 2}-0, exhibits a non-classical primary deuterium kinetic isotope effect on its Arrhenius activation energy ({Delta}G{sup TS}), where {Delta}G{sup TS} for the protiated form is larger than for the deuterated form. The same behavior is observed during steady state turnover of the cyt bc{sub 1} complex using ubiquinol, but not plastoquinol, as a substrate, leading to the conclusion that similar chemical pathways are involved in both systems. The synthetic system is an unambiguous n=1 electron acceptor and it is thus inferred that sequential oxidation of ubiquinol (by two sequential n=1 processes) is more rapid than a truly concerted (n=2) oxidation in the cyt bc{sub 1} complex.
- Research Organization:
- Pacific Northwest National Lab., Richland, WA (US), Environmental Molecular Sciences Laboratory (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15015964
- Report Number(s):
- PNWD-SA-6728; JACSAT; 2358; TRN: US0501691
- Journal Information:
- Journal of the American Chemical Society, Vol. 127, Issue 12; Other Information: PBD: 30 Mar 2005; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ACTIVATION ENERGY
BINDING ENERGY
CYTOCHROMES
DEUTERIUM
ELECTRON TRANSFER
ELECTRONS
ISOTOPE EFFECTS
KINETICS
MAGNETIC RESONANCE
OXIDATION
PROBES
PROTONS
REACTION INTERMEDIATES
TRANSIENTS
VALENCE
ENVIRONMENTAL MOLECULAR SCIENCES LABORATORY
CYTOCHROME BC1 COMPLEX
QO SITE
UBIQUINOL
RUTHENIUM
TRANSITION STATE
PROTON COUPLED ELECTRON TRANSFER
CONCERTED ELECTRON TRANSFER
SEMIQUINONE
TIME-RESOLVED ELECTRON PARAMAGNETIC RESONANCE
KINETIC ISOTOPE EFFECT