Elucidation of the Structure of Lignin–Carbohydrate Complexes in Ginkgo CW-DHP by (13)C-(2)H Dual Isotope Tracer

To elucidate the chemical linkages between lignin and carbohydrates in ginkgo cell walls, (13)C-(2)H-enriched cell wall-dehydrogenation polymers (CW-DHP) were selectively prepared with cambial tissue from Ginkgo biloba L. by feeding D-glucose-[6-(2)H(2)], coniferin-[α-(13)C], and phenylalanine ammonia-lyase (PAL) inhibitor. The abundant detection of (13)C and (2)H confirmed that D-glucose-[6-(2)H(2)] and coniferin-[α-(13)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 (13)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 (13)C-NMR and (1)H-NMR differential spectroscopy of (13)C-(2)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 (13)C-(2)H-enriched GML fraction. The formation of ketal linkages between the C-α of lignin and xylan was confirmed in the (13)C-(2)H-enriched XL fraction.