Structural Characterization of Lignin and Lignin-Carbohydrate Complex (LCC) from Ginkgo Shells (Ginkgo biloba L.) by Comprehensive NMR Spectroscopy

Lignin and lignin-carbohydrate complexes are important polymers for lignocellulosic biorefinery and functional materials, but those in ginkgo shells are not effectively analyzed and exploited. Based on this background, milled wood lignins (MWLML and MWLFZ) and lignincarbohydrate complexes (LCCML and LCCFZ) were isolated from the shells of Ginkgo biloba L. cv. Damaling (ML) and Ginkgo biloba L. cv. Dafozhi (FZ) correspondingly, and were structurally characterized by comprehensive NMR spectroscopy. The results showed that ginkgo shells exhibited higher lignin (42%) and xylan (20%) content than general softwood species. Isolated MWLs were rich in guaiacyl units with the presence of ferulates and p-coumarates, and the molecular formula was C9H7.93O2.73(OCH3)(0.81) and C9H7.87O2.76(OCH3)(0.88) for MWLML and MWLFZ, respectively. Phenolic hydroxyl of MWLML (1.38 mmol/g) and MWLFZ (1.23 mmol/g) in ginkgo shells was much less than that in general softwoods, suggesting a higher etherification and condensation degree of ginkgo shells lignin, and beta-5', alpha-O-4', and 4-O-5' bonds were the main condensed structures. O-acetylated beta-D-xylopyranoside and beta-D-mannopyranoside were the main polysaccharides associated with lignin, and the acetyl groups frequently acylate the C-2 and C-3 positions. LCCML had more phenyl glycoside (0.035/Ar) and less gamma-ester (0.026/Ar) linkages than LCCFZ.