Elucidating tricin-lignin structures : assigning correlations in HSQC spectra of monocot lignins

Tricin [5,7-dihydroxy-2-(4-hydroxy-3,5-dimethoxyphenyl)-4H-chromen-4-one] is a flavone that has been found to be incorporated in grass lignin polymers via 4′–O–β coupling. Herein, we investigated the tricin-lignin structure using nuclear magnetic resonance (NMR) methods by comparing the 1H–13C heteronuclear correlation (HSQC) NMR spectra of the isolated lignin with a series of dimeric and trimeric tricin-4′–O–β-ether model compounds. Results showed that the tricin moiety significantly affects the chemical shift of the Cβ/Hβ of 4′–O–β unit, producing peaks at around δC/δH 82.5–83.5/4.15–4.45, that differ from the Cβ/Hβ correlations from normal 4–O–β units formed solely by monolignols, and that have to date been unassigned.
The authors thank the China Scholarship Council, State Education Department, for supporting living expenses for Wu Lan’s Program in the Department of Biological System Engineering, University of Wisconsin, Madison. W.L., F.Y., F.L., and J.R. (John Ralph) were funded by the DOE Great Lakes Bioenergy Research Center (DOE BER Office of Science DE-FC02-07ER64494 and DE-SC0018409). J.R. (Jorge Rencoret) and J.C.d.R. were funded by the Spanish Projects CTQ2014-60764-JIN and AGL2017-83036-R (co-financed by FEDER funds).