Detection and Determination of p-Coumaroylated Units in Lignins

The d erivatization f ollowed by r eductive c leavage (DFRC) method cleaves α- and β-ethers in lignins but leaves lignin γ-esters intact. When applied to grasses, which contain p -coumarate esters on their lignins, esterified monolignol derivatives are released. Saturation of the p -coumarate double bond occurs during DFRC, so the released products are 4-acetoxycinnamyl 4-acetoxyphenylpropionates. Synthesis of the esters allowed determination of response factors for the released products. Maize and bamboo lignins released 221 and 38 μmol/g of p -coumarate-derived esters. The sinapyl ester was much more abundant than the coniferyl one. The bamboo and maize lignin S/G ratios in the conjugates were 12 and 38 times greater than those of the normal monomers released by DFRC, evidence of a strong selectivity for acylation of syringyl units. Of three possible biochemical mechanisms for incorporating p -coumarates into lignin, evidence is mounting that the process involves incorporation of preacylated monolignols into the normal lignification process.