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

By combining mild alkaline hydrolysis with quantitative P-31 NMR we have been able to arrive at a protocol for determining the various ester linkages and their relative contributions to the overall structure of wheat straw lignin. Additional information on the identity and location of these bonds was sought by the application of GC/MS and two-dimensional C-13-H-1 heterocorrelation NMR experiments. Milled straw lignin was found to contain about 12 ester units per 100 phenylpropane 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 application of C-13-H-1 correlative NMR experiments revealed that acylation occurs only at the gamma-position of the lignin side chain. Detailed studies of two-dimensional HOHAHA and HMQC experiments failed to show evidence for the presence of alpha-O-4 substructures in milled wheat straw lignin.