Pathway of p-Coumaric Acid Incorporation into Maize Lignin as Revealed by NMR

NMR methods can be used to delineate detailed structural and regiochemical information on the plant cell wall and elucidate biochemical incorporation pathways. Maize lignin isolated in high yield from rind tissue of stem internodes contained high amounts of esterified p-coumaric acid. Available literature indicated that acylation of the lignin by p-coumaric acid was at the αor γ-position of the lignin side-chain, implicating two diverse biochemical pathways. Application of 13C–1H correlative NMR experiments to this maize lignin, synthetic αand γ-p-coumaroylated lignin model compounds, and a specifically labeled synthetic coniferyl alcohol/coniferyl p-coumarate dehydrogenation polymer (DHP) unambiguously revealed that p-coumaric acid is attached exclusively at the γ-position. The biochemical implication is that lignin acylation occurs by enzymatically controlled pre-acylation of lignin monomers which are subsequently incorporated into the lignin polymer via oxidative coupling. “Opportunistic” acylation at the α-position via quinone methide intermediates is insignificant. Fig. 1. Plant cell wall model compounds; p-hydroxycinnamic acids 1, monomeric p-hydroxycinnamyl alcohol lignin precursors 2, a general structure of lignin 3, quinone methide lignin intermediates 4, p-hydroxycinnamyl p-coumarates 5, and αand γ-phydroxycinnamate ester model compounds 6-8 synthesized for NMR correlation work. For compounds 1, 2, 4, and 5; a R = R' = H, b R = OCH 3 , R' = H; c R = R' = OCH 3 . O O HO O O