Structural characterisation by MALDI-MS of olive xylo-oligosaccharides obtained by partial acid hydrolysis

Alkali extractable olive pulp and olive seed hull glucuronoxylans were submitted to mild acid hydrolysis originating low molecular weight xylo-oligosaccharides. The xylo-oligosaccharides thus formed were partially separated by size exclusion chromatography (Biogel P6) and several elution fractions were identified by MALDI-MS. The MALDI mass spectra obtained revealed the presence of several xylo-oligosaccharides ions in each fraction between the m/z ranges 500–2500. Based on the sugar and methylation analyses of the glucuronoxylans, the proposed structures are the neutral xylo-oligosaccharides (Xyln, n=4–18) and acidic xylo-oligosaccharides substituted by glucuronic acid residues (XylnGlcA, n=3–16 and XylnGlcA2, n=2–14), by 4-O-methyl-glucuronic acid residues (XylnMeGlcA, n=3–14 and Xyl2MeGlcA2) and by both acid residues (XylnGlcAMeGlcA, n=4–7). The identification of ions corresponding to long blocks of neutral and acidic oligosaccharides suggested that the acidic residues were irregularly distributed along the xylan backbone in both olive pulp and olive seed hull. In addition, MALDI mass spectra showed that the acidic xylo-oligosaccharides from the pulp were predominantly constituted by XylnMeGlcA oligosaccharides, while the acidic xylo-oligosaccharides from the seed hull were predominantly constituted by XylnGlcA and XylnGlcA2 oligosaccharides.