EPR Evidence for hydroxyl- and substrate-derived radicals in Fe(II)-oxalate/hydrogen peroxide reactions. The importance of the reduction of Fe(III)-oxalate by oxygen-conjugated radicals to regenerate Fe(II) in reactions of carbohydrates and model compounds

Abstract

EPR spectroscopy has been employed for the direct detection of a variety of free radicals formed from reaction of Fe(II)–oxalate and H₂O₂ in the presence of carbohydrates and related compounds: this system has been designed to model the proposed mode of action of brown rot fungi. The observed hyperfine splittings allow characterization of individual radicals formed at different positions in the carbohydrate rings. Relative signal intensities in steady-state spectra indicate the rapid generation of the hydroxyl radical, followed by relatively unselective attack of ˙OH on the substrates’ C–H bonds: the rapidity of oxidation by Fe(III) of oxygen-conjugated carbon-centred radicals (typically k 10⁸ dm³ mol^–1 s^–1) is significantly reduced if there is an eclipsing β-oxygen substituent. The relevance of these findings to cellulose-cleaving reactions of certain fungi is discussed.