Journal article
Thermodynamic and kinetic considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide
Free radical biology & medicine, Vol.49(6), pp.919-962
2010
DOI: 10.1016/j.freeradbiomed.2010.05.009
PMCID: PMC2936108
PMID: 20493944
Abstract
The quinone/semiquinone/hydroquinone triad (Q/SQ
•−/H
2Q) represents a class of compounds that has great importance in a wide range of biological processes. The half-cell reduction potentials of these redox couples in aqueous solutions at neutral pH,
E°′, provide a window to understanding the thermodynamic and kinetic characteristics of this triad and their associated chemistry and biochemistry
in vivo. Substituents on the quinone ring can significantly influence the electron density “on the ring” and thus modify
E°′ dramatically.
E°′ of the quinone governs the reaction of semiquinone with dioxygen to form superoxide. At near-neutral pH the p
K
a's of the hydroquinone are outstanding indicators of the electron density in the aromatic ring of the members of these triads (electrophilicity) and thus are excellent tools to predict half-cell reduction potentials for both the one-electron and two-electron couples, which in turn allow estimates of rate constants for the reactions of these triads. For example, the higher the p
K
a's of H
2Q, the lower the reduction potentials and the higher the rate constants for the reaction of SQ
•− with dioxygen to form superoxide. However, hydroquinone autoxidation is controlled by the concentration of di-ionized hydroquinone; thus, the lower the p
K
a's the less stable H
2Q to autoxidation. Catalysts, e.g., metals and quinone, can accelerate oxidation processes; by removing superoxide and increasing the rate of formation of quinone, superoxide dismutase can accelerate oxidation of hydroquinones and thereby increase the flux of hydrogen peroxide. The principal reactions of quinones are with nucleophiles via Michael addition, for example, with thiols and amines. The rate constants for these addition reactions are also related to
E°′. Thus, p
K
a's of a hydroquinone and
E°′ are central to the chemistry of these triads.
Details
- Title: Subtitle
- Thermodynamic and kinetic considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide
- Creators
- Yang Song - College of Pharmaceutical Sciences, Key Laboratory of Eco-environments in Three Gorges Reservoir Region, Ministry of Education, Southwest University, Chongqing, 400715, People's Republic of ChinaGarry R Buettner - Free Radical and Radiation Biology Program, The University of Iowa, Iowa City, IA 52242-1181, USA
- Resource Type
- Journal article
- Publication Details
- Free radical biology & medicine, Vol.49(6), pp.919-962
- DOI
- 10.1016/j.freeradbiomed.2010.05.009
- PMID
- 20493944
- PMCID
- PMC2936108
- NLM abbreviation
- Free Radic Biol Med
- ISSN
- 0891-5849
- eISSN
- 1873-4596
- Publisher
- Elsevier Inc
- Language
- English
- Date published
- 2010
- Academic Unit
- Radiation Oncology; Iowa Superfund Research Program
- Record Identifier
- 9984047781602771
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