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[35] Characterization of o-semiquinone radicals in biological systems

1990; Academic Press; Linguagem: Inglês

10.1016/0076-6879(90)86127-h

1557-7988

Balaraman Kalyanaraman,

Electron Spin Resonance Studies

o-Semiquinone radicals are intermediates in the one-electron oxidation of catechols, catecholamines, and catechol estrogens. This chapter discusses the characterization of o-semiquinone radicals in biological systems. It discusses the principle of kinetic stabilization of o-semiquinone using the catecholamine dopa as an example. An ultraviolet photolysis of neutral solutions of dopa gives the steady-state electron spin resonance (ESR) spectrum, which is typical of dopa semiquinone. Time-resolved experiments show that the radical is transient and decay with second-order kinetics under these conditions. The radical decay corresponds to a second-order rate constant of 2.5 x 108 M–1 sec–1, close to that reported for the semiquinone from epinephrine. The results obtained are markedly different when complexing metal ions are included in the reaction mixture. Using Zn2+ ions at pH 5.0, the only radical detected is the semiquinone complexed with Zn2+. The complexed radicals are much less transient than the uncomplexed ones. Decay remains second order, but the radical lifetime is now several seconds for a steady state concentration of approximately 10–5 M. The calculated second order rate constant is 1.1 x 104 M–1 sec–1 for these conditions. Chelation is, therefore, extremely effective in decreasing the rate of radical termination. The uncomplexed o-semiquinone at neutral pH has a rate constant over 10,000-fold greater. Thus, the complexed radical can be detected at rates of radical formation 10,000 times lower than are necessary to detect the uncomplexed o-semiquinone. This allows the use of static rather than flow systems.