Elucidation of the Structure of Lignin-Carbohydrate Complexes in Ginkgo CW-DHP by C-H Dual Isotope Tracer.

To elucidate the chemical linkages between lignin and carbohydrates in ginkgo cell walls, C-H-enriched cell wall-dehydrogenation polymers (CW-DHP) were selectively prepared with cambial tissue from L. by feeding D-glucose-[6-H], coniferin-[α-C], and phenylalanine ammonia-lyase (PAL) inhibitor. The abundant detection of C and H confirmed that D-glucose-[6-H] and coniferin-[α-C] were involved in the normal metabolism of ginkgo cambial cells that had been effectively labelled with dual isotopes. In the ginkgo CW-DHP, ketal and ether linkages were formed between the C-α of lignin side chains and carbohydrates, as revealed by solid state CP/MAS C-NMR differential spectroscopy. Furthermore, the DMSO/TBAH ionic liquids system was used to fractionate the ball-milled CW-DHP into three lignin-carbohydrate complex (LCC) fractions: glucan-lignin complex (GL), glucomannan-lignin complex (GML), and xylan-lignin complex (XL). The XRD determination indicated that the cellulose type I of the GL was converted into cellulose type II during the separation process. The molecular weight was in the order of Ac-GL > Ac-GML > XL. The C-NMR and H-NMR differential spectroscopy of C-H-enriched GL fraction indicated that lignin was linked with cellulose C-6 by benzyl ether linkages. It was also found that there were benzyl ether linkages between the lignin side chain C-α and glucomannan C-6 in the C-H-enriched GML fraction. The formation of ketal linkages between the C-α of lignin and xylan was confirmed in the C-H-enriched XL fraction.