BIOMIMETIC DEGRADATION OF LIGNIN

The ligninolytic cultures of Phenarochaete chrysosporium produce two major groups of lignin degrading enzymes, the lignin peroxidase (Tien and Kirk, 1983) and the manganese-dependent peroxidase (Kuwahara et al., 1984). Both enzymes are hemeproteins and catalyze the oxidation of lignin model compounds by one-electron oxidations. The catalytic cycle of both enzymes involves the two-electron oxidation of iron(III) protoporphyrin IX, the prosthetic group of both enzymes, by hydrogen peroxide to give the highly reactive oxoiron(IV) protoporphyrin IX pi-cation radical, which returns to its resting state after two separate one-electron reductions by the substrates. The oxidation of simple metalloporphyrins can give similar highly oxidized species corresponding to the catalytic intermediates of the hemeprotein peroxidases. In the case of iron porphyrins (Groves et al., 1979; Chin et al., 1977) the oxidized intermediate is an oxoiron(IV) porphyrin cation radical and in the case of manganese porphyrins, the oxidized species is an oxomanganese(V) porphyrin (Groves et al., 1980). Shimada et al. (1977, 1984) have used commercially available metalloporphyrins as models of the lignin degrading enzymes. We have synthesized and used metalloporphyrins 1-4 (see Fig. 2) as biomimetic catalysts for lignin oxidation (Dolphin et al., 1987; Cui and Dolphin, 1989; Cui, 1990). The chloro substituents on the phenyl rings provide steric protection to increase the stability of the porphyrins (1-4) towards excess oxidants and the chlorines on the porphyrin periphery make 3 and 4 more powerful catalysts by increasing the redox potential at the metal centre.