Laccase catalysed modification of lignin subunits and coupling to p-aminobenzoic acid

Laccase catalysed oxidation of syringyl and guaiacyl subunits of lignin and their modification with an aromatic amine, p-aminobenzoic acid (PABA) were investigated. Laccase from Galerina sp. HC1 isolated earlier by us was used as the main catalyst, and Trametes versicolor laccase was used for comparison. Among the syringyl compounds, syringic acid and syringaldehyde were oxidised to 2,6-dimethoxy-1,4-benzoquinone, and in the presence of PABA yielded a cross-coupling imine product. The reaction with methyl syringol resulted in several products whose structures were determined. The possible oxidative coupling pathways were proposed for the formation of the identified products. Oxidation of syringol and the guaiacyl compounds resulted mainly in homooligomers by free radical mechanism, with a negligible tendency of reaction with the nucleophilic group of PABA. Similar treatment of Eucalyptus Kraft lignin, which is rich in syringyl moieties, showed the presence of identical products obtained with syringic acid and syringaldehyde.