Application of NMR spectroscopy to structural studies of lignin: Dissertation

The structures of isolated and synthetic lignin preparations were studied by two- and three-dimensional (2D and 3D) nuclear magnetic resonance (NMR) spectroscopy. 2D HMQC and 3D HMQC-TOCSY NMR experiments provided better resolution than conventional 1D NMR experiments where signals are broad and overlapping, and allowed less ambiguous assignments of the interunit linkages. The recently discovered prominent linkages in softwood lignins, the trans- and cis-dibenzodioxocins, were identified in 13C-enriched poplar milled wood lignin (MWL), suspension cell culture lignin (RSCL) and dehydrogenated polymer (DHP) preparations. Only the trans-dibenzodioxocin was found in non-enriched milled reed canary grass lignin and in pine MWL preparations. The poplar lignin preparation also contained small amounts of non-cyclic a-aryl ether linkages. In addition to the improved detection of minor components, the 3D HMQC-TOCSY experiment provides a tool to chart out the spin systems of unknown structural units in a sample. New spin systems were discovered in the 3D NMR spectra of 13C-enriched lignin samples and one of them was identified as corresponding to a neolignan with a spiro skeleton. This structure, a possible precursor of the b-1 linkage, was detected in 13C-labelled poplar MWL, RSCL and DHP preparations. The dehydrogenative polymerization of b-13C-labelled coniferyl alcohol was studied to find out if preformed DHP could function as a template and induce a structural pattern closely resembling a natural softwood lignin. The experiments were performed at pH 4 and pH 6.5 and the distribution of structural units was estimated in 1D and 2D NMR experiments. Comparison of b-13C-labelled DHP with the unlabelled polymer showed that the pH has a stronger effect than the added DHP on the manner of coupling of coniferyl alcohol.