Hydroxystilbenes Are Monomers in Palm Fruit Endocarp Lignins

Lignin, the plant cell wall polymer that binds fibers together but makes processing difficult, is traditionally formed from three monomers, the so-called monolignols (p-coumaryl, coniferyl, and sinapyl alcohols). Recently, we discovered, in grass lignins, a phenolic monomer that falls outside the canonical lignin biosynthetic pathway, the flavone tricin. As we show here, palm fruit (macaúba [Acrocomia aculeata], carnauba [Copernicia prunifera], and coconut [Cocos nucifera]) endocarps contain lignin polymers derived in part from a previously unconsidered class of lignin monomers, the hydroxystilbenes, including the valuable compounds piceatannol and resveratrol. Piceatannol could be released from these lignins upon derivatization followed by reductive cleavage, a degradative method that cleaves β-ether bonds, indicating that at least a fraction is incorporated through labile ether bonds. Nuclear magnetic resonance spectroscopy of products from the copolymerization of piceatannol and monolignols confirms the structures in the natural polymer and demonstrates that piceatannol acts as an authentic monomer participating in coupling and cross-coupling reactions during lignification. Therefore, palm fruit endocarps contain a new class of stilbenolignin polymers, further expanding the definition of lignin and implying that compounds such as piceatannol and resveratrol are potentially available in what is now essentially a waste product.