Substrate radical, electron paramagnetic resonance

When one looks for methods to detect OH, one always has two keep in mind that these radicals are very reactive, and in the presence of substrates their steady-state concentrations are extremely low even at a high rate of OH production. The fact that OH only absorbs far out in the UV region (Hug 1981) is thus not the reason why an optical detection of OH is not feasible. Electron paramagnetic resonance (EPR) must also fail because of the extremely low steady-state concentrations that prevail in the presence of scavengers. The only possibility to detect their presence is by competition of a suitable OH probe that allows the identification of a characteristic product [probe product, reaction (41)]. When this reaction is carried out in a cellular environment, the reaction with the probe is in competition with all other cellular components which also readily react with OH [reaction (42)]. The concentration of the probe product is then given by Eq. (43), where [ OH ] is the total OH concentration that has been formed in this cellular environment and q is the yield of the probe product per OH that has reacted with the probe. 

Tetramethylpyrazine (365) generated the hexamethylpyrazine radical cation (366), sufficiently persistent to yield an excellent electron paramagnetic resonance (EPR) spectrum (mixed within the EPR cavity substrate, Me2S04, Zn or Bu 4NBH4, PhH) homologues likewise.184... 

Zhao, Y., Abend, A., Kunz, M., Such, P., and Retey, J., 1994, Electron paramagnetic resonance studies of the methylmalonyl-CoA mutase reaction. Evidence for radical intermediates using natural and artificial substrates as well as the competitive inhibitor 3-carboxypropyl-CoA, Eur. J. Biochem. 225 8919896. 

Yl. Yamazaki, I., Mason, H. S., and Piette, L., Identification, by electron paramagnetic resonance spectroscopy, of free radicals generated from substrates by peroxidase. J. Biol. Chem. 236, 2444-2449 (1960). 

Although there may be one or two exceptions, there is no evidence, as revealed by kinetics, effects of anti-oxidants, electron paramagnetic resonance spectral data, etc. for the belief that dioxygenases exploit this form of catalysis. Unfortunately, precedents for bona fide non-radical chain catalyses are hard to find. Nevertheless, several examples are known of oxidation of ligands and substrates by dioxygen co-ordinated to metal. 

Patton and West studied the electrochemistry of these species. The radical anions of squarate 2, croconate 3, and rhodizonate 4 were characterized in dichloromethane using the electron paramagnetic resonance (EPR) technique [17]. Likewise, Carr, Fabre, and collaborators obtained the UV/visible and EPR spectra of these radical dianions, produced electrochemically in dimethylfor-mamide [18]. The oxidation potential of the oxocarbonic acids was determined in perchloric acid solution using platinum electrodes. The oxidative process was proposed to proceed in two stages, beginning with the transfer of charge from substrates at the electrode. Subsequently, the oxidation product is desorbed from the electrode and hydrated [19]. 

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