Unfortunately, the complexity of the lignin polymer makes it difficult to study microbial ligninolysis. The lack of well-characterized model substrates that can be used to identify ligninolytic reactions in vivo has been a long-standing difficulty for mechanistic studies (Crawford 1981).
Dimeric model compounds that represent the principal substructures of lignin have been used successfully to characterize the ligninolytic systems of white rot fungi. Models of this type provided some of the first evidence that P. chrysosporium and T. versicolor cleave the lignin isopropyl side chain between Ca and Cp (Crawford 1981). Dimeric models played a large role in revealing that fungal lignin peroxidase (LIP) cleaves lignin between Ca and Cp, which represent 7% of the linkages in the lignin polymer (Gold et al. 1989; Hammel et al. 1993; Kirk and Farrell 1987). Furthermore, oxidation of a p-O-4 model compound, which represents 50-60% of the bonds within the lignin molecule demonstrated that LIP can cleave the predominance of linkages in lignin (Glenn et al. 1983; Tien and Kirk 1983a,b). The main product is the corresponding benzaldehyde. Dimeric models have also been used to detect LIP activity in situ in fungus-colonized wood, where extraction and conventional assay of the enzyme is technically difficult (Srebotnik et al. 1994).
Degradation of a (p-O-4)-(5-50) type trimer, arylglycerol-p-(dehydrodivanillyl alcohol) ether (I) by LIP showed that during its degradation by the enzyme, Ca-Cp cleavage, p-O-4 bond cleavage, and p-etherified aromatic ring (B-ring) opening products were formed (Umezawa and Higuchi 1989; Yokota et al. 1991). The results showed that the B-ring of substrate (I), which must be sterically hindered more than those of arylglycerol-p-guaiacyl and arylglycerol-p-(2,6-dimethoxyphenyl) ethers was oxidized by LIP (Umezawa and Higuchi 1989; Yokota et al. 1991).
Dimeric lignin model compounds have the disadvantage of low molecular weight. Unlike lignin, they can be taken up and metabolized intracellularly by microorganisms, which can make it difficult to determine whether the degradation products observed really reflect ligninolytic activity (Crawford 1981). Ideally, lignin model compounds should be macromolecular like lignin, but to facilitate product analysis, they should have simpler structures than those of the natural polymer. For this reason dimeric lignin compounds attached to a polymer backbone such as polystyrene and polyethylene glycol have been developed (Kawai et al. 1995).
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