Enzymatic modification of kraft lignin through oxidative coupling with water-soluble phenols

The aromatic polymer lignin can be modified through promotion of oxidative coupling between phenolic groups on lignin and various phenols. The reactio n is initiated by an oxidation of both components, e.g., by using the oxido reductases laccase or peroxidase. Coupling between phenolic monomers and li gnin has previously been studied by the use of radio-labeled phenols. In th is study, incorporation of water-soluble phenols into kraft lignin, using l accase as catalyst, was investigated. Several phenols with carboxylic or su lfonic acid groups were used as markers for the incorporation. The modified lignin was isolated and the amount of phenol incorporated was characterize d by means of titration, quantitative H-1-NMR, and quantitative P-31-NMR af ter modification with 2-chloro-4,4.5,5-tetramethyl-1,2,3-dioxaphospholane. Only a few of the phenols studied were found to be incorporated into lignin . When the phenol guaiacol sulfonate was incorporated into kraft lignin, th e lignin became water-soluble at pH 2.4 and a low ionic strength due to the introduction of sulfonic acid groups. The content of sulfonic acid groups in the product was 0.5-0.6 mmol/g lignin. A lower amount of C-hydroxy pheny lacetic acid was incorporated under similar conditions.