An Aspect of the Lignin Problem (II)

One of the principal remaining aspects of the lignin problem which is required for the ultimate elucidation of the chemical structure of natural lignin is knowledge of the exact nature of the linkage or linkages which join the phenylpropane building stones together in the structure of the lignin. To be sure, many investigations have been made on the nature of these linkages, but the evidence on which they are based has been of a rather indirect type. A wide variety of chemical reactions have been applied to lignin in an effort to chemically degrade the polymer into dimeric or other higher oligomeric fragments, thereby sheding light on the nature of the linkages of phenylpropane units in lignin itself. But the chemical degradative reactions employed are too vigorous, and destroy the linkages which is to be studied.An alternate approach has been suggested in which the “depolymerization” of lignin would be accomplished enzymically rather chemically. White-rot fungi and other micro-organisms are adapted to lignin utilization by cultivating these fungi in a lignin-containing medium for a prolonged period. The principal degradation products formed from the lignin by the result of a fungal attack were vanillin, vanillic acid, dehydrodivanillin, ferulic acid, coniferaldehyde, 4-hydroxy-3-methoxyphenylpyruvic acid, guaiacylglycerol, and guaiacylglycerol-β-coniferyl ether. β-Hydroxyconiferyl alcohol and 4-hydroxy-3-methoxyphenylpyruvic acid, which are formed from guaiacylglycerol groups present in guaiacylglycerol-β-aryl ether units of softwood lignin by the enzymic degradation, were converted to vanillin by the activity of fungal laccase or peroxidase. Vanillin may be more easily metabolized via vanillic acid and protocatechuic acid by enzymic reactions. Therefore, the quantity of degradation products accumulated in a lignin-containing culture medium was usually small. The open vanillin-yielding moieties of lignin were preferentially attacked during its enzymic degradation by these enzymes. Many investigators have been presented an evidence which indicates that the polymerization of coniferyl alcohol derivatives to lignin-like materials (DHP) is catalyzed by a laccase or peroxidase in the presence of hydrogen peroxide. The present data suggest that aromatic compounds, such as vanillyl alcohol, vanillyl ethyl ether, β-hydroxyconiferyl alcohol and 4-hydroxy-3-mehtoxyphenylpyruvic acid, are converted to vanillin by an oxidative cleavage of the side chains as a function of above enzyme. While the formation of the polymers from other phenolic compounds employed occured by dehydrogenation polymerization as the result of a enzymic reaction of peroxidase.Although many investigations have been reported on the biochemical degradation of lignin and on the enzymic reactions involved, some evidence on the intermediary phases of this process is only available. To gain a better understanding of the mechanism of the enzymic degradation of lignin by fungi, the studies on the degradation products of lignin and related compounds, and on the properties of many enzymes required may be effectively carried out in the nearly future.