Detection and Determination of p-Coumaroylated Units in Lignins

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Abstract
The derivatization followed by reductive cleavage (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. Keywords: Acetyl bromide; lignin; grass; Gramineae; maize; bamboo; bromegrass; p-coumarate; phenolic acid; β-aryl ether; cleavage; quantitative analysis; gas chromatography; reductive elimination; acetylation; bromination; DFRC