Lignin structure has been considered to be an important factor that significantly influences the biorefinery processes. In this work, the effect of ball milling on the structural components and extractable lignin in enzymatic residues was evaluated, and the structural characteristics of the cellulolytic enzyme lignin preparations isolated from wheat straw stem (SCEL) and leaf (LCEL) were comparatively investigated by a combination of nitrobenzene oxidation (NBO), ozonation, infrared spectroscopy, and 1 H− 13 C heteronuclear single quantum coherence nuclear magnetic resonance (2D HSQC NMR). The results showed that 4 h ball-milled samples were good enough for structural analysis with high lignin yield. Both CELs are typical p-hydroxyphenylguaiacyl-syringyl lignins which are associated with pcoumarates and ferulates. However, the structure of lignin in wheat straw stem is rather different from that in leaf. Compared to stem lignin, leaf lignin has lower product yields of NBO and ozonation, lower erythro/threo ratio, and higher condensation degree. The analysis of 2D HSQC NMR indicated that the S/G ratio of SCEL was 0.8, which is about twice as much as that of LCEL. The flavone tricin is incorporated into both stem and leaf lignins. The content of tricin in LCEL is higher than that in SCEL.

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The applications of quartz crystal microbalance (QCM) rely heavily on the preparation of ultrathin films. So far, techniques on direct lignocellulosic film making without components isolation have been hardly investigated. In this work, a novel approach was developed to prepare spin-coated ultrathin films based on the complete dissolution of ball-milled wood in LiCl/DMSO solvent system. The surface analysis and elemental composition of the films respectively using atomic force microscopy and X-ray photoelectron spectroscopy proved that an even-textured lignocellulosic film could be formed on QCM gold sensors. The prepared ultrathin films were successfully applied on monitoring the enzymatic hydrolysis process in situ and in real time by QCM. The changes of QCM frequency showed clearly that the enzymatic hydrolysis of lignocellulosic materials could be divided into three stages, including cellulase adsorption, fast substrate hydrolysis, and slow substrate hydrolysis. The adsorption and hydrolysis processes were fitted with Lagergren and Boltzmann-sigmoidal kinetic models, respectively, indicating that cellulase adsorption on lignin and cellulose is competitive and that lignin inhibits the enzymatic hydrolysis of cellulose.

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The isolation of lignin is of great importance to understand its structural characteristics. A lithium chloride/dimethyl sulfoxide (LiCl/DMSO) solvent system has been developed for the dissolution of lignocellulose and for the isolation of lignin for this purpose. In this work, ball-milled wheat leaf (sheath included) was dissolved in the LiCl/DMSO solvent system and then regenerated in water. Two lignin preparations, cellulolytic enzyme lignin from the ball-milled leaf (CEL) and from the regenerated leaf (RCEL), were obtained through a cellulolytic enzyme lignin procedure. The RCEL and CEL were comparatively investigated by the use of wet chemistry and spectroscopic methods. The results indicate that the effects of ball milling and regeneration on the aromatic structure and β-O-4' linkages of lignin were not significant. The RCEL had a higher isolation yield and purity, but a similar structure with the corresponding CEL. The RCEL can be used for structural analysis.