Analysis of the structure of lignin -: carbohydrate complexes by the specific 13C tracer method

In the present study the specifically C-13-enriched lignin precursors of biosynthesis (i.e., coniferin-[side chain-alpha-C-13], coniferin-[side chain-beta-C-13] and coniferin-[side chain-gamma-C-13]) were synthesized and administered exogeneously to ginkgo shoots (Ginkgo biloba L.) to obtain C-13-enriched lignin-carbohydrate complexes (LCCs), The specifically C-13-enriched LCCs were isolated from the newly formed xylem of ginkgo shoots administered with the C-13-enriched precursors and degraded by enzymes. Lignin-rich fractions, so called enzyme-degraded LCCs (EDLCCs), were obtained. By determining their C-13-NMR spectra, information related to the chemical structure of lignin building units and linkages between phenylpropane units of lignin and carbohydrates were obtained. It was found that these precursors were incorporated in natural lignin successfully. Three lignin-carbohydrates linkages (i.e., ether type, ester type, ketal type) were found at the C-alpha-position of the side chain of phenylpropane units in ginkgo LCC, No lignin-carbohydrate bond at the C-beta- or C-gamma-position of the lignin side chain was observed in the C-13-NMR spectra of the C-13-enriched LCCs. This fact indicates that a specific C-13 tracer technique can be useful in NMR study of the chemical structure of LCCs.