2D-NMR (HSQC) difference spectra between specifically ¹³C-enriched and unenriched protolignin of Ginkgo biloba obtained in the solution state of whole cell wall material

In the structural analysis of lignins by 13 C-NMR, signal overlap limits definitive assignment and accurate intensity measurement. Selective labeling by 13 C-enrichment of a specific carbon in lignin enhances its signal intensity in the spectrum. Further enhancement of the specifically labeled carbons can be realized via difference spectra created from the enriched and unenriched samples. Difference 2D 13 C- 1 H correlation (HSQC) NMR spectra, derived from the spectra of specifically 13 C-enriched lignin model polymers (so-called dehydrogenation polymers) and their unenriched counterparts, take advantage of the enhanced dispersion afforded by both 13 C and 1 H chemical shifts, diminishing the difficulties arising from the signal-overlap problem and aiding in definitive signal assignments. In this research, protolignin in xylem cell walls was specifically 13 C-enriched at all of the individual phenylpropanoid side-chain carbons by feeding 13 C-enriched coniferins to growing stems of Ginkgo biloba . The whole xylem fractions containing 13 C-enriched and unenriched protolignins were dissolved in a mixture of N-methylimidazole and DMSO, and then acetylated. Solution state 2D-NMR (HSQC) spectra of the acetylated whole cell wall were acquired. Difference spectra between the walls containing 13 C-enriched and unenriched lignins afforded simplified 2D spectra in which well-separated signals were assigned exclusively to the specifically enriched carbons. This novel NMR technique provides a useful tool for elucidation of entire protolignin in the cell wall of ginkgo xylem.