Homogeneous Polymerization of Kraft Lignin Using an Alkaliphilic Multi-Copper Oxidase (Bilirubin Oxidase) in a Borate Buffer

Enzymatic modification of Kraft lignin under alkaline conditions was investigated using bilirubin oxidase (BOD) in borate buffer (pH 10). Control solubilization without enzyme addition revealed a notable increase in molar mass (up to 1.7-fold) and potential borate complexation with lignin hydroxyl groups, as evidenced by thermogravimetric and ¹¹B NMR analyses. BOD treatments induced substantial polymerization, with molar mass increases of up to 4-fold for insoluble fractions after 24 h, while soluble fractions exhibited progressive increases over 5 days. Quantitative ³¹P NMR showed reductions in aliphatic and phenolic hydroxyl groups by 20%, suggesting oxidative coupling reactions, particularly through 4-O-5' and 5-5' linkages. Solid-state ¹³C NMR confirmed structural changes associated with polymerization. Dynamic light scattering (DLS) indicated the presence of colloidal aggregates, potentially explaining challenges in HSQC NMR signal acquisition. These findings highlight the efficacy of bilirubin oxidase in catalyzing lignin polymerization and underscore the structural impact of borate-lignin interactions in alkaline media, paving the way for advanced lignin valorization strategies.