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Probes of the Radical Site

The involvement of a free radical in the active site of galactose oxidase is not obvious and was, in fact, overlooked for many years. As mentioned earher (Section V, C), the Cu(II) ion strongly interacts with the radical in the active enzyme (AGO), and the normal spectroscopic signatures of a free radical are absent. The strong interactions between the radical and the metal ion result in an EPR-silent complex and an unusual absorption spectrum that is clearly not a simple superposition of spectra for Cu(II) and a free radical (see below). However, a distinctive and unusual free radical EPR signal is consistently present as a minority species in EPR spectrum of oxidant-treated GAOX. This same feature is produced when [Pg.28]

This is markedly different from the behavior of a simple tyrosine phenoxyl, such as that found in ribonucleotide reductase, whose spectrum exhibits a strong rhombic splitting (Fig. 17, line c) but precisely the same as observed for the 0-methylthiocresyl model radical (Fig. 17, line b). This clearly identihes the Tyr-Cys side chain as the site of the oxidized apoGAOX radical and demonstrates that the electronic structure of the thioether-substituted phenoxyl is distinct from that of a simple phenoxyl radical. [Pg.32]

The SOMO/LUMO gap is large for ere, but there are hlled valence levels lying at slightly deeper binding energy. [Pg.34]

The predicted electronic delocalization onto the thioether side chain in the mtc phenoxyl free radical ground state has been conhrmed experimentally by EPR measurements on mtc specihcally deuterated in the side [Pg.34]

The effect of suUur participation on the orbital g -shifts in the EPR spectra, illustrated in Pig. 20, accounts for the qualitatively different spectra observed for tyrosyl phenoxyl and Tyr-Cys phenoxyl radicals (Gerfen et al., 1996). The rhombicity of the simple tyrosyl radical EPR spectrum is a consequence of the splitting between gx and gy principal g -values. These g -shifts deviate from the free electron g--value ge = 2.00023) as a result of orbital angular momentum contributions. While a nondegenerate electronic state (such as the A ground state for ere) contains no hrst-order unquenched orbital momentum, second-order spin-orbit mixing between close-lying a and a functions results [Pg.35]


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