Oxygen paramagnetic intermediates

The presumed H202 intermediate appears to decompose by l-e steps, as a paramagnetic intermediate is formed with laccase. With 170 this has been shown to represent an oxygen radical, possibly Or, having unusual relaxation properties. 

Reactions with H2O2 may be divided into two classes arising from the homolytic vy. heterolytic cleavage of the 0-0 bond (173). In homolytic catalysis, the oxygen-centered radicals are intermediates the participation of concerted processes in heterolytic catalysis precludes paramagnetic intermediates. Product selectivity is usually higher in the latter class. Transition metal cations in low oxidation states, such as Cu, Ti, Cr, and Fe " ", catalyze the homolytic... 

Working with more limited amounts of oxygen it becomes apparent that there are additional stable paramagnetic intermediates preceeding the formation of 0. The semidione of can be detected but it is very sensitive to traces of oxygen. When prepared from the acetoxy ketone (Scheme 9) is the only paramagnetic... 

On adding dioxygen to the fully reduced laccase of the lacquer tree Rhus vemicifera, the type-1 Cu and the type-3 Cu-pair were oxidized in the ms range and an optical intermediate was observed at 360 nm At liquid helium temperatures an EPR signal was observed, which was tentatively interpreted as due to O ", as a result of its very short relaxation time and of the increase of its linewidth when the reduced laccase of the fungus Polyporus versicolor was treated with 0 A similar paramagnetic oxygen intermediate was also observed with the laccase of another lacquer tree Rhus succedanea and with ceruloplasmin. The decay of the intermediate at 25 °C (tj = 1 s at pH 5.5 with R. succedanea laccase) was accompanied by the reoxidation of the type-2 Cu >. One would expect, however, such an intermediate to be extremely reactive (See Sect. 3.3), while it was stable in tree laccase depleted of type-2 Cu(II)... 

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