Isolation of Cellulolytic Enzyme Lignin from Rice Straw Enhanced by LiCl/DMSO Dissolution and Regeneration

Abstract

Ball-milled rice straw was dissolved in a lithium chloride/dimethyl sulfoxide (LiCl/DMSO) solvent system, regenerated, and subjected to enzymatic hydrolysis to obtain regenerated cellulolytic enzyme lignin (RCEL). The structure of the isolated lignin was characterized by elemental analysis, gel permeation chromatography (GPC), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and proton nuclear magnetic resonance (1H NMR). Alkaline nitrobenzene oxidation (NBO) was conducted to analyze the structural characteristics of the in-situ lignin. The results showed that the rice straw RCEL was composed of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) phenylpropane units, with relatively high amounts of H units. The yield of RCEL is about 5% units higher than that of cellulolytic enzyme straw lignin (CEL) on the basis of total lignin in the original rice straw. When compared to the CEL obtained by the traditional method, there were no observed differences versus RCEL in terms of the elemental compositions, NBO product yields, and S/G ratio. The weight-average molecular weight of RCEL was 6835, which was lower than that of CEL, indicating that some rice straw lignin linkages were cleaved during LiCl/DMSO dissolution.