Biomimetic Studies of Lignin Degradation and Bleaching
Cui, F., David Dolphin, Wijesekera, D., Farrell, R., and Skerker, P.
Applications of Biotechnology of Pulp and Paper Manufacture, Eds. T.K. Kirk and H.-M. Chang., Butterworth-Heinemann, Boston, Massachusetts, U.S.A.
Unlike most other biopolymers such as proteins, polysaccharides and polynucleotides, which are made up of discrete monomers linked by a single repeating functional group, lignin is a three-dimensional, hetero- geneous polymer composed of phenylpropanoid units. Because of its complex structure and stable C-C and C-O linkages connecting its con- stituting units, the biodegradation of this polymer must differ from the degradation of other biopolymers. Indeed, the lignin degrading enzymes, lignin peroxidases (Iigninases) and manganese-dependent peroxidases,9 isolated from Phanerochaete Chrytoaporium are non-specific peroxidases that catalyze the degradation of lignin, or at least lignin model com- pounds, by one-electron oxidations. It is unlikely that lignin degrading enzymes will have active sites which can bind specific substructures of lignin and oxidize lignin at their active sites. The question then arises as to how a heterogeneous lignin molecule interacts with these enzymes and electron carriers (mediators) may be important. While manganese complexes are effective mediators for the oxidation of lignin model com- pounds by the manganese-dependent peroxidase, no effective mediators have so far been discovered for the lignin peroxidases. Although it has been suggested that the cation radical of veratryl alcohol, or other small molecules, could be mediators of lignin degradation, the far more important role of veratryl alcohol was found later to be protection of the enzyme from inactivation. Simple metalloporphyrins, when sterically protected from self de- struction, can mimic the lignin degrading enzymes. They can be even more powerful catalysts than the enzymes when they are electronically activated. Being small molecules, porphyrin catalysts have an additional advantage in that they can readily interact with lignin without the need of mediators. Commercially available porphyrins have been used as ligninase models and found to be able to catalyze a variety of reactions with lignin model compounds. However, the porphyrin complexes used were unstable in the presence of excess oxidants and could not mimic all the ligninase catalyzed reactions. A series of stable, water soluble porphyrins have been synthesized in our laboratory and in this paper we compare their catalytic activities and discuss the reactions of lignin and lignin model compounds catalyzed by them.