ESR Spectra of Mechanically Formed Free Radicals

Identification of ESR Spectra of Mechanically Formed Free Radicals 

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This is usually isolated from yeast, and has a molecular weight of 53 000 with one heme b. It catalyzes the oxidation of ferrocytochrome c by hydrogen peroxide. It is the first peroxidase for which the structure has been determined. The imidazole axial ligand is His-174, while Arg-48, Trp-51 and His-52 provide distal catalytic groups. The mechanism involves nucleophilic attack of peroxide on the Fe, loss of the ROOH proton to the imidazole of His-52, and transfer of this proton to the leaving RO group. Compound I of cytochrome c peroxidase is red, and differs from HRPI in that the additional oxidizing equivalent is on a protein residue. ESR and ENDOR spectra have been interpreted in terms of a methionine-centred free radical. One possibility is that a ferryl porphyrin cation radical is formed with cytochrome c peroxidase (it is attractive to assume that this would be common to all peroxidases), but that cytochrome c peroxidase has a readily oxidizable substrate which reduces the porphyrin radical. ... 

ESR Spectra. The question had been raised as to what type of polymerization mechanism was involved. We propose a free-radical process on the basis of the analogy between the cyclic bis(arylene tetra-sulfides) and sulfur itself. To resolve this point, the polymerization of monomer I, in bulk, was carried out in the cavity of an ESR spectrometer. The ESR signal shown in Figure 1 clearly demonstrates a free-radical mechanism, and the g-value (spectroscopic splitting factor) of 2.023 for the free radicals formed during polymerization agrees closely with the g-value of 2.024 reported for polymerization of sulfur (24). 

The mechanism of pyrrole interaction with acylacetylenes has been investigated using the spin-capture technique [504], Based on the analysis of the ESR spectra, it can be suggested that ion-radical pair B, formed in the reaction under stndy, is not a stable intermediate species in nonpolar medium. Proton transfer from the cation radical of pyrrole to the anion radical of benzoylacetylene occurs, and free radicals thus generated are captnred by the spin trap (ONBu-t) (Scheme 2.46). 

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