Stereospecificity in enzymic and non-enzymic oxidation of beta-O-4 lignin model compounds

The degradation of the erythro and threo isomers of the non-phenolic lignin model compound 2-(2,6-dimethoxyphenoxy)-1-(3,4-dimethoxyphenyl)-1,3-propanediol was examined. Enzymic and non-enzymic oxidation of the diastereomers was performed with Trametes versicolor lignin peroxidase and cerium(IV) ammonium nitrate, respectively. Mixtures of approximately equal amounts of the diastereomers were partially degraded and subsequently analyzed with TLC and 1H-NMR. Analysis of reaction mixtures from enzymic as well as non-enzymic oxidation, revealed a preferential degradation of the threo form. Preliminary analyses of enzymic reaction mixtures of either the erythro or threo isomer suggest they yield in part different products. The observations made would have implications for the understanding of how enzymes attack lignins. They should also be taken into consideration in experiments where model compounds are being used to mimic native lignin.