Demonstration of the presence of acetylation and arabinose branching as structural features of locust bean gum galactomannans

Galactomannans from locust bean gum (LBG) are polysaccharides containing single Galp residues as side chains in a β-(1 → 4)-Manp backbone. Nevertheless, sugars other than mannose and galactose are always detected, namely, arabinose and glucose, although in small amounts. In order to demonstrate that LBG galactomannans contain other structural features than those usually reported in literature, the galactomannans from LBG obtained from two different commercial sources were submitted to a selective degradation by Aspergillus niger endo-β-d-mannanase and the oligosaccharides (OS) obtained were fractioned by size exclusion chromatography using a Biogel-P2 (100–1800 Da). A. niger endo-β-d-mannanase is able to hydrolyse the β-(1 → 4)-linked mannan backbone when the mannose residues are not hindered by the presence of substituents. This enables collection of fragments of the substituted mannan polysaccharide in the different chromatographic fractions that can be analysed by Electrospray Tandem Mass Spectrometry (ESI-MSn). Both samples showed the presence of hexose OS (Hex3–6), acetylated hexoses (AcHex3–6), hexoses substituted with a pentose unit (PentHex3–6), and OS composed by acetylated hexoses substituted with a pentose unit (PentAcHex3–6). These structures were confirmed by GC–qMS of reduced and permethylated oligosaccharides. Sugar and glycosidic-linkage analysis allowed to assign the pentose unit to a terminally-linked arabinofuranose residue at the O-6 of a (1 → 4)-Manp residue. The presence of arabinose substituted OS and acetylated mannoses have never been reported to occur in LBG.