On the role of 1-hydroxybenzotriazole as mediator in laccase oxidation of residual kraft lignin

In order to elucidate the role of the mediator 1-hydroxybenzotriazole (HBT) in laccase oxidation processes, vanillyl alcohol was oxidized with laccase in the presence and in the absence of this mediator. The pattern of metabolites produced showed that the presence of HBT allowed the formation of oxygen addition products such as o-and p-quinones and aromatic ring cleavage products, while in its absence only oxidative coupling products were observed. Experiments carried out on residual kraft lignin showed the occurrence of coupling reactions during laccase oxidation. In the presence of the laccase-mediator (LM) system the reaction pathway was driven toward side-chain oxidation and oxygen addition products, while the formation of condensed structures was depressed. Furthermore, The role of phenolic, aliphatic OH, and COOH moieties in lignin degradation was elucidated by selectively blocking them and then submitting the samples to laccase and laccase-HBT treatments. It was found that the activity of the LM system depends on the presence of phenolic lignin subunits. In actual fact, the reactions carried out on lignins lacking free phenolic groups did not cause any appreciable degradation to the polymer.