Structural analysis of wheat straw lignin by quantitative 31P and 2D NMR spectroscopy; The occurrence of ester bonds and a-O-4 substructures

By combining mild alkaline hydrolysis with quantitative 31P NMR we have been able to arrive at a protocol for determining the various ester linkages and their relative contributions to the overall structureofwheatstrawlignin. Additionalinformationontheidentityandlocationofthesebonds 13 1 was sought by the application of GC/MS and two-dimensional C- H heterocorrelation NMR experiments. Milledstrawligninwasfoundtocontainabout12esterunitsper100phenylpropane units. Approximately 77% of the carboxyl fraction of these ester bonds was found to be composed of p-coumaric acid while the rest was other aromatic acids bound to lignin via intra- and/or intermolecular ester bonds. In contrast, the hydroxyl fraction of the ester bonds was found to be almost exclusively aliphatic. A small fraction (about 1.6%) of the milled straw lignin units was found to be esterified through the phenolic hydroxyl groups of C-5 condensed phenolic units. The 13 1 application of C- H correlative NMR experiments revealed that acylation occurs only at the γ-position of the lignin side chain. Detailed studies of two-dimensional HOHAHA and HMQC experiments failed to show evidence for the presence of R-O-4 substructures in milled wheat straw lignin.