Fig. 2. Chemical structure of lignin determined using solid-state and liquid NMR.

Lignin signals are resolved using the aromatic regions of a 2D ¹³C-¹³C RFDR spectra and b 2D ¹³C J-INADEQUATE spectra of intact plant cells of eucalyptus, poplar, and spruce. c The structure of lignin units and linkages resolved in wood stems. The standard abbreviations are given under each structure. The observed ¹³C chemical shifts are labeled for each carbon site. Key inter-residual linkages are highlighted. d Composition of lignin units quantified using the integrals of one-bond cross peaks in 2D ¹³C RFDR spectra (left panel) and using solution NMR as reported in the literature (right panel)^{30,32,33,35,73,85,86}. Eucalyptus and poplar are S-rich while spruce is G-rich. Source data are provided as a Source Data file. e Representative lignin linkages detected by 2D ¹H-¹³C HSQC spectra of ball-milled biomass in d₆-DMSO.