An ESR spin trapping investigation on the photoreduction of chlorohemin in mixed solvents

Radical scavenging by nitrone compounds is used in conjunction with ESR spectroscopy in order to obtain evidence for radical formation in the photoreduction of chlorohemin in aqueous alcoholic mixed solvents. The formation of adducts between hydrogen atoms and the spin traps appears to be the most important process in aerated and in pyridine-containing solutions. Evidence for the formation of hydroxyethyl adducts is obtained when the alcoholic component of the solvent is ethanol. In deaerated solutions not containing pyridine, no evidence is obtained for radical formation. The results are interpreted in terms of hydrogen abstraction by the nitrone from an alkoxy radical formed together with Fe(II) in the primary photoreduction. The abstraction process strongly competes with cage re-oxydation, as well as with the diffusion of the alkoxy radical in the solvent bulk where it would rapidly react to give hydroxy radicals. The role of pyridine and oxygen may respectively be stabilizing and oxidizing the Fe(II) species before it undergoes cage re-oxidation. © 1983.