Identification of ferulate oligomers from corn stover

BACKGROUND: Cross-links between plant cell wall polymers negatively impact forage digestibility. Hydroxycinnamates and their oligomers act as cross-links between polysaccharides and/or polysaccharides and lignin. Higher ferulate oligomers such as dehydrotrimers were identified in cereal grains but not in vegetative organs of grasses. The aim of this study was to characterize ester-linked hydroxycinnamate oligomers from corn stover with special emphasis on ferulate dehydrotrimers. RESULTS: With the exception of the 4-O-5-dehydrodiferulic acid all known ferulate dehydrodimers, including the recently described 8-8(tetrahydrofuran) dimer, were identified in the alkaline hydrolyzate of corn stover after chromatographic fractionation. Next to dehydrodimers, 18 cyclobutane dimers made up of ferulic acid and/or p-coumaric acid were identified by GC-MS of the dimeric size exclusion chromatography fraction. Ferulate dehydrotrimers were isolated by using multiple chromatographic procedures and identified by UV spectroscopy, MS and NMR. Four trimers were unambiguously identified as 5-5/8-O-4-, 8-O-4/8-O-4-, 8-8(aryltetralin)/8-O-4-, and 8-O-4/8-5-dehydrotriferulic acids, a fifth tentatively as 8-5/5-5-dehydrotriferulic acid. CONCLUSION: The formation of ferulate dehydrotrimers is not limited to reproductive organs of grasses but also contribute to network formation in the cell walls of vegetative organs. Although radically coupled hydroxycinnamate dimers and oligomers were in the focus of researchers over the last decade, the earlier described cyclobutane dimers significantly contribute to cell wall cross-linking.