PATHWAY OF P-COUMARIC ACID INCORPORATION INTO MAIZE LIGNIN AS REVEALED BY NMR

NMR methods can be used to delineate detailed structural and regiochem ical information on the plant cell wall and elucidate biochemical inco rporation pathways. Maize lignin isolated in high yield from rind tiss ue 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 alpha- or gamma-position of the lignin side chain, implicating two diverse biochemical pathways. Application of C- 13-H-1 correlative NMR experiments to this maize lignin, synthetic alp ha- and gamma-p-coumaroylated lignin model compounds, and a specifical ly labeled synthetic coniferyl alcohol/coniferyl p-coumarate dehydroge nation polymer (DHP) unambiguously revealed that p-coumaric acid is at tached exclusively at the gamma-position. The biochemical implication is that lignin acylation occurs by enzymatically controlled pre-acylat ion of lignin monomers which are subsequently incorporated into the li gnin polymer via oxidative coupling. ''Opportunistic'' acylation at th e alpha-position via quinone methide intermediates is insignificant.