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

Abstract

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.