An NMR Comparison of the Whole Lignin from Milled Wood, MWL, and REL Dissolved by the DMSO/NMI Procedure

Lignins isolated from pine milled wood, milled wood lignin (MWL), and residual enzyme lignin (REL) were compared using modified thioacidolysis, modified DFRC, gel permeation chromatography (GPC), two-dimensional Heteronuclear Multiple Quantum Coherence (HMQC) NMR, and quantitative C NMR. Dissolution of the lignin for solution-state NMR was accomplished by utilizing the recently reported DMSO/N-methylimidazole/acetic anhydride solvent system. Contrary to previous reports, comparison of the lignin preparations by thioacidolysis indicated that REL was more structurally similar to the lignin in the milled wood and Wiley wood meal than MWL. Total monomer yields indicated that the MWL was lower in b-aryl ether content than the other preparations, and this was verified by quantitative C NMR. NMR analysis indicated that the inter-unit linkages present in all the lignin preparations are consistent with the present knowledge about lignin biosynthesis. The contribution of minor end group structures in the MWL are further decreased in the milled wood, indicating that they are preferentially isolated as low molecular weight material, possibly generated during the milling process. All other structural moieties were similar in all preparations. GPC data indicated that the milled wood and REL both contain a portion of lignin with a molecular weight of 55,000 g/mol. Data Address correspondence to John F. Kadla, Faculty of Forestry, Department ofWood Science, University of British Columbia, 2424 Main Mall, Vancouver, BC, Canada V6T 1Z4. E-mail: [email protected] Journal of Wood Chemistry and Technology, 27: 179–200, 2007 Copyright # Taylor & Francis Group, LLC ISSN 0277-3813 print/1532-2319 online DOI: 10.1080/02773810701700828