Evaluation of ring-5 structures of guaiacyl lignin in Ginkgo biloba L. using solid- and liquid-state 13C NMR difference spectroscopy

To discuss the macromolecular structure and properties of lignin, the complementary use of solid- and liquid-state nuclear magnetic resonance (NMR) combined with a specific ¹³C-enrichment technique may provide useful information. The ¹³C-enriched lignin was prepared by administering [guaiacyl ring-5(G5)-¹³C]-coniferin to a growing Ginkgo biloba L. shoot. The ¹³C-enriched cellulolytic enzyme lignin (EL) and its acetate prepared from the ginkgo shoot were examined by solid- and liquid-state ¹³C NMR spectroscopy. The ¹³C NMR spectrum derived only from the G5 carbon was obtained as a difference spectrum based on the spectra of the G5-¹³C enriched and un-enriched (UE) samples. The condensed structures, including the enriched G5-carbon, were evaluated using difference spectra. The chemical shifts of the enriched G5 carbon in both the solid- and liquid-state ¹³C NMR spectra agreed with each other in principle. The quantitative ratio of the condensed and uncondensed structures at G5 was found to be larger by solid-state cross polarization/magic angle spinning (CP/MAS) NMR than by liquid-state NMR.