Fig. 6: Pectin-lignin interactions identified in inflorescence stems.

a Resolved signals of carbonyl sites from acetyl groups (-OCOMe, purple) present in both xylan and pectin, as well as methyl esters (-COOMe, blue) and carboxylates (-COO⁻, orange) in pectin, observed in CP-based refocused J-INADEQUATE spectra. The representative chemical structure and corresponding chemical shifts of these key carbon sites are also provided. b Specific cross-peak regions extracted from 2D dipolar-gated ¹³C-¹³C correlation spectra with a long-mixing time of 1 s. The purple band highlights correlations between lignin aromatic carbons and acetyl groups. Yellow and blue bands indicate lignin interactions with pectin carboxylates and methyl esters, respectively. c Structural representation illustrating intermolecular interactions between lignin aromatic and carbonyl sites in xylan (blue) and the GalA units in HG (yellow). Note that GalA residues may also originate from RG-I. Black dashed lines represent intermolecular interactions shown in (b), while solid lines indicate intramolecular correlations observed in the spectra in (a). d Molar fraction of methylated GalA estimated by comparing the peak volumes of the resolved signals corresponding to methylated and unmethylated GalA units in spectra similar to the one shown in (a). e Cross-peaks between methyl/carbonyl groups and other carbohydrate carbons observed in a 2D dipolar-gated spectrum with a 0.1 s mixing time. NMR abbreviations are used for rhamnose (R), 3-fold xylan (Xn^3f), 2-fold xylan (Xn^2f), and galacturonic acid (GA). Dashed arrows highlight chemical shifts from the C6 of rhamnose (R6; 18.5 ppm), acetyl methyl (AcMe; 21 ppm), methylated GalA C6 (GA6^Me; 171 ppm), acetyl carbonyl (AcCO; 173.5 ppm), and unmethylated GalA C6 (GA6; 176 ppm). Dashed boxes in green, blue, and orange highlight the integration regions used for estimating the distribution of acetyl groups in different carbohydrates. f Molar distribution of acetyl groups in Xn^2f, Xn^3f, and GalA, respectively. Source data are provided as a Source data file.