The effect of laccase on cellulase-treated lignin in 1-n-butyl-3-methylimidazolium chloride/H2O homogeneous and H2O heterogeneous solutions

Abstract

In this study, the effect of laccase earlier obtained by our laboratory on cellulase-treated lignin (CEL) in two different solution systems was further investigated. Results obtained were as follows: After laccase treatment of CEL in the heterogeneous water solution, CEL was then compared with control sample A. Ultraviolet (UV) spectra showed that the total absorbance of sample B increased at 205 and 280 nm; Fourier-transform infrared (FTIR) spectra showed that the carbonyl group obviously appeared; and C-13 nuclear magnetic resonance (NMR) spectra showed that the C-gamma signals disappeared, the C-alpha signals significantly increased, and the methoxyl content decreased. After laccase treatment of CEL in the [bmim]Cl/H2O homogeneous solution, CEL was compared with control sample C. UV spectra showed that the total absorbance of sample D decreased at 280 nm; high-performance liquid chromatography (HPLC) analysis showed that some small absorption peaks disappeared; and C-13 NMR spectra showed that the C-gamma signals obviously increased, the C-alpha signals slightly decreased, while the methoxyl content increased. Clearly, for insoluble CEL in the heterogeneous water solution, laccase partly oxidized the hydroxyl group into a carbonyl group, and thereby partly degraded CEL to increase its solubility. In contrast, for soluble CEL in the [bmim]Cl/H2O homogeneous solution, laccase primarily polymerized small molecule fragments with the CEL macromolecule, increased the methoxyl content of CEL, and thereby played a role in lignin polymerization.