Sugar beet (Beta vulgaris) pectins are covalently cross-linked through diferulic bridges in the cell wall

Arabinan and galactan side chains of sugar beet pectins are esterified by ferulic acid residues that can undergo in vivo oxidative reactions to form dehydrodiferulates. After acid and enzymatic degradation of sugar beet cell walls and fractionation of the solubilized products by hydrophobic interaction chromatography, three dehydrodiferulate-rich fractions were isolated. The structural identification of the different compounds present in these fractions was performed by electrospray-ion trap-mass spectrometry (before and after (18)O labeling) and high-performance anion-exchange chromatography. Several compounds contained solely Ara (terminal or alpha-1-->5-linked-dimer) and dehydrodiferulate. The location of the dehydrodiferulate was assigned in some cases to the O-2 and in others to the O-5 of non-reducing Ara residues. One compound contained Gal (beta-1-->4-linked-dimer), Ara (alpha-1-->5-linked-dimer) and dehydrodiferulate. The location of the dehydrodiferulate was unambiguously assigned to the O-2 of the non-reducing Ara residue and O-6 of the non-reducing Gal residue. These results provide direct evidence that pectic arabinans and galactans are covalently cross-linked (intra- or inter-molecularly) through dehydrodiferulates in sugar beet cell walls. Molecular modeling was used to compute and structurally characterize the low energy conformations of the isolated compounds. Interestingly, the conformations of the dehydrodiferulate-bridged arabinan and galactan fragments selected from an energetic criterion, evidenced very nice agreement with the experimental occurrence of the dehydrodiferulated pectins. The present work combines for the first time intensive mass spectrometry data and molecular modeling to give structural relevance of a molecular cohesion between rhamnogalacturonan fragments.