Lignin–feruloyl ester cross-links in grasses. Part 1. Incorporation of feruloyl esters into coniferyl alcohol dehydrogenation polymers
Methyl 5-O-(E)-[γ-13C]feruloyl-α-L-arabinofuranoside (FA-Ara) has been synthesized and incorporated into a synthetic lignin dehydrogenation polymer (DHP) of coniferyl alcohol. Inverse-detected long-range C–H correlation NMR experiments on the DHP lignin gave correlation peaks indicative of the copolymerization of the FA-Ara and coniferyl alcohol into the DHP polymer. The bonding sites and modes, as determined by analysis of the carbonyl region of the long-range C–H correlated 2D NMR experiment, are predictable from free-radical coupling mechanisms. In addition to the abundant 4-O-α′ and 4-O-β′ ether couplings, structures involving the β-position of the feruloyl moiety of FA-Ara in β-ether, phenylcoumaran and pinoresinolide structures were present. The incorporation of feruloyl esters into a lignin DHP results in some structures which would not release ferulic acid by solvolytic schemes currently used for quantitation of ferulic acid in plant materials. Thus the degree to which hydroxycinnamic acids are involved in the lignification of forages may be significantly underestimated.