Understanding the degree of condensation of phenolic and etherified C-9 Units of in situ lignins

Abstract

A novel approach for the quantification of the degree of condensation at the C₅ position of etherified and phenolic phenylpropane (C-9) units of in situ lignin is described. This is achieved by degrading unmethylated and methylated wood by thioacidolysis and analyzing the resultant product mixtures by quantitative ³¹P NMR spectroscopy. Applying this new method to compression wood and normal wood from Pinus radiata showed that, whereas 41–47% of etherified guaiacyl C-9 units are condensed at the C₅ position, almost all phenolic guaiacyl C-9 units exist as uncondensed moieties. Analysis of milled wood lignin (MWL) isolated from the same wood by ³¹P NMR spectroscopy before and after thioacidolysis showed that the phenolic guaiacyl C-9 units were more condensed than those in the in situ lignin. This is likely due to partial cleavage of the more condensed etherified linkages during the lignin isolation, leading to a relative increase in condensed phenolic guaiacyl C-9 units.