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 (MWL(ML) and MWL(FZ)) and lignin-carbohydrate complexes (LCC(ML) and LCC(FZ)) 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 C(9)H(7.93)O(2.73)(OCH(3))(0.81) and C(9)H(7.87)O(2.76)(OCH(3))(0.88) for MWL(ML) and MWL(FZ), respectively. Phenolic hydroxyl of MWL(ML) (1.38 mmol/g) and MWL(FZ) (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 β-5′, α-O-4′, and 4-O-5′ bonds were the main condensed structures. O-acetylated β-d-xylopyranoside and β-d-mannopyranoside were the main polysaccharides associated with lignin, and the acetyl groups frequently acylate the C(2) and C(3) positions. LCC(ML) had more phenyl glycoside (0.035/Ar) and less γ-ester (0.026/Ar) linkages than LCC(FZ).