BIOMIMETIC DEGRADATION OF LIGNIN

The ligninolytic cultures of Phenarochaete chrysosporium produce two m ajor groups of lignin degrading enzymes, the lignin peroxidase (Tien a nd Kirk, 1983) and the manganese-dependent peroxidase (Kuwahara et al. , 1984). Both enzymes are hemeproteins and catalyze the oxidation of l ignin model compounds by one-electron oxidations. The catalytic cycle of both enzymes involves the two-electron oxidation of iron(III) proto porphyrin IX, the prosthetic group of both enzymes, by hydrogen peroxi de to give the highly reactive oxoiron(IV) protoporphyrin IX pi-cation radical, which returns to its resting state after two separate one-el ectron reductions by the substrates. The oxidation of simple metallopo rphyrins can give similar highly oxidized species corresponding to the catalytic intermediates of the hemeprotein peroxidases. In the case o f iron porphyrins (Groves et al., 1979; Chin et al., 1977) the oxidize d intermediate is an oxoiron(IV) porphyrin cation radical and in the c ase of manganese porphyrins, the oxidized species is an oxomanganese(V ) porphyrin (Groves et al., 1980). Shimada et al. (1977, 1984) have us ed commercially available metalloporphyrins as models of the lignin de grading enzymes. We have synthesized and used metalloporphyrins 1-4 (s ee 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 t he porphyrin periphery make 3 and 4 more powerful catalysts by increas ing the redox potential at the metal centre.