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

In the present study the specifically C-13-enriched lignin precursors of bi osynthesis (i.e., coniferin-[side chain-alpha-C-13], coniferin-[side chain- beta-C-13] and coniferin-[side chain-gamma-C-13]) were synthesized and admi nistered exogeneously to ginkgo shoots (Ginkgo biloba L.) to obtain C-13-en riched lignin-carbohydrate complexes (LCCs), The specifically C-13-enriched LCCs were isolated from the newly formed xylem of ginkgo shoots administer ed with the C-13-enriched precursors and degraded by enzymes. Lignin-rich f ractions, so called enzyme-degraded LCCs (EDLCCs), were obtained. By determ ining their C-13-NMR spectra, information related to the chemical structure of lignin building units and linkages between phenylpropane units of ligni n and carbohydrates were obtained. It was found that these precursors were incorporated in natural lignin successfully. Three lignin-carbohydrates lin kages (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 cha in was observed in the C-13-NMR spectra of the C-13-enriched LCCs. This fac t indicates that a specific C-13 tracer technique can be useful in NMR stud y of the chemical structure of LCCs.