Grafting of mediators by laccases: A potential path to upgrade lignin

Lignin consists of a three dimensional mesh of aromatic structures and is currently an underexploited resource in biorefining of plant biomass.  Laccases (EC 1.10.3.2) principally catalyse oxidation of phenolic hydroxyls using O₂ as electron acceptor, but have also been suggested to induce bond cleavage between phenolic units in lignin in the presence of mediators acting as electron vehicles. This paper reports enzyme-assisted grafting of -N-OH type mediators on lignin using laccases from Trametes versicolor and Pleurotus ostreatus, respectively. The study assesses the laccase facilitated reactivity of different mediators HBT (1-hydroxybenzotriazole), HPI (N-hydroxyacetanilide), TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxy), and ABTS (2,2’-azinobis(3-ethylbenzthiazoline-6-sulfonate)). The two laccases were equally efficient in catalysing the grafting of the -N-OH type mediators, i.e. HBT and HPI, to lignin, but HPI appeared to graft 7-10 times better than HBT. Lignin obtained from exhaustive cellulase treatment of wheat straw was more susceptible to grafting than beech organosolv lignin (mediator to lignin ratio was 35% vs. 11% for HPI and 5% vs. 1% for HBT on the wheat straw lignin and organosolv lignin, respectively). All laccase-mediator treatments promoted oxidation in subunits of the wheat straw lignin, with TEMPO and ABTS being most efficient, and the mediator influenced the selectivity of modified subunits. The overall ratio of syringyl-like units to guaiacyl-like units did not change upon reaction. The data imply that lignin can be functionalized via laccase-assisted catalysis.