Identification of the structure and origin of a thioacidolysis marker compound for ferulic acid incorporation into angiosperm lignins (and an indicator for cinnamoyl CoA reductase deficiency)

Abstract

A molecular marker compound, derived from lignin by the thioacidolysis degradative method, for structures produced when ferulic acid is incorporated into lignin in angiosperms (poplar, Arabidopsis, tobacco), has been structurally identified as 1,2,2-trithioethyl ethylguaiacol [1-(4-hydroxy-3-methoxyphenyl)-1,2,2-tris(ethylthio)ethane]. Its truncated side chain and distinctive oxidation state suggest that it derives from ferulic acid that has undergone bis-8-O-4 (cross) coupling during lignification, as validated by model studies. A diagnostic contour for such structures is found in two-dimensional (13)C-(1)H correlated (HSQC) NMR spectra of lignins isolated from cinnamoyl CoA reductase (CCR)-deficient poplar. As low levels of the marker are also released from normal (i.e. non-transgenic) plants in which ferulic acid may be present during lignification, notably in grasses, the marker is only an indicator for CCR deficiency in general, but is a reliable marker in woody angiosperms such as poplar. Its derivation, together with evidence for 4-O-etherified ferulic acid, strongly implies that ferulic acid is incorporated into angiosperm lignins. Its endwise radical coupling reactions suggest that ferulic acid should be considered an authentic lignin precursor. Moreover, ferulic acid incorporation provides a new mechanism for producing branch points in the polymer. The findings sharply contradict those reported in a recent study on CCR-deficient Arabidopsis.