Exploring Alkyl-O-Alkyl Ether Structures in Softwood Milled Wood Lignins

Recent studies have suggested that there are significant amounts of various alkyl ether (Alk-O-Alk; Alk = alkyl) moieties in a spruce native lignin preparation, milled wood lignin (SMWL). However, the comprehensive NMR assignment to these moieties has not been addressed yet. This study focused on investigating different types of Alk-O-Alk structures at the alpha-and gamma-positions of the lignin side chain in an heteronuclear single-quantum coherence (HSQC) spectrum of SMWL using experimental NMR data of lignin and synthesized model compounds. Ambiguous structural features were predicted by computer simulation of 1H and 13C NMR spectra to complement the experimental NMR data. As a result, specific regions in the HSQC spectrum were attributed to different Alk-O-Alk moieties of Alk-O-Alk/beta-O-4 and Alk-O-Alk/beta-beta ' structures. However, the differences between the specific regions were rather subtle; they were not well separated from each other and some major lignin moieties. Furthermore, SMWL contained a large variety of Alk-O-Alk moieties but in minute individual amounts, resulting in rather broad, superimposing resonances. Thus, evaluation did not allow assigning individual types of Alk-O-Alk moieties from the HSQC spectra; instead, they were quantified as total (alpha-and gamma-linked) Alk-O-Alk based on the balance of structural units in the 13C NMR spectra. At last, potential formation mechanisms of various Alk-O-Alk ether structures in lignin biosynthesis, lignin aging, and during ball milling of wood were and discussed.

Automated summary

Recent studies have identified significant amounts of various alkyl ether moieties in milled wood lignin (SMWL), which have not been fully characterized. This study used experimental NMR data and computer simulation to analyze the different types of alkyl ether structures in SMWL. Specific regions in the HSQC spectrum were attributed to different alkyl ether moieties, but the differences were subtle and difficult to separate from other lignin moieties. Additionally, the quantification of individual alkyl ether types was challenging, and the study discussed potential formation mechanisms of alkyl ether structures in lignin biosynthesis, aging, and ball milling of wood.