Quantification of Wheat Straw Lignin by Comprehensive NMR Techniques

Quantification of wheat straw lignin by comprehensive NMR techniques

Jijiao Zeng¹, Greg Helms² and Shulin Chen^1*

1. Department of Biological Systems Engineering, Washington State University, Pullman, Washington 99164-6120, USA

2. Department of Chemistry, center for NMR spectroscopy, Washington State University, Pullman, Washington 99164-6120, USA

^*Corresponding author. Tel: +1 509 335 3743; fax: +1 509 335 2722.

E-mail address: chens@wsu.edu

 

Abstract:

Lignin is secondary natural organic compositions of plant cell wall besides cellulose and hemicelluse. Structurally lignin is linked with hemicelluloses and cellulose to form recalcitrant complex that resist the environmental stress. Aiming to increase efficiency of carbohydrate saccharification, many efforts have been made to remove and/or modify lignin moieties. However, due to limitation of our knowledge on detection and quantification of lignin unit at structural level, the relationship between structural changes and influence on cellulase after these processes is still unclear. In order to provide complete structural information of lignin units, a quantitative method was developed to characterize wheat straw lignin subunits by combination of quantitative ¹³C NMR and HSQC 2D NMR techniques. Milled wood lignin (MWL) and cellulolytic enzyme-treated lignin (CEL) was isolated from wheat straw and characterized by 500 MHz NMR spectrometer. In addition to common syringyl, guaiacyl and p-hydroxyphenol units, various units and side chains including b-O-4, dibenzodioxocin, phenylcoumaran, pinoresinol, spirodienone ferrulate, coumarate and p-benzonate were identified and quantified. The results also showed that CEL contained more lignin-carbohydrate linkages (LCC) and polysaccharide anomeric compared to MWL. Moreover, the increased amount of ferrulate in CEL illustrated that wheat straw lignin was cross-linked with polysaccharides by ferrulate unit which may limit polysaccharide digestibility in non-woody biomass. p-benzonate and p-coumarate are both identified as acetylated group in ¦Ã-position of lignin side chains. The approach taken and the information obtained in the study will provide useful information for elucidating mechanisms of pretreatment technologies in terms of changes caused to lignin units.