The free reducing oligosaccharides of gum arabic: aids for structural assignments in the polysaccharide

Gum arabic contains in small quantities, the free reducing sugars, arabinose, ribose, glucose, mannose, galactose, rhamnose, α-d-Galp-(1→3)-αβ-l-Ara, α-l-Araf-(1→4)-β-d-Galp-(1→6)-αβ-d-Gal, β-d-GlcpA-(1→6)-αβ-d-Galp, α-l-Rhap-(1→4)-β-d- GlcpA-(1→6)-αβ-d-Galp, α-l-Rhap-(1→4)-β-d-GlcpA-(1→6)-β-d-Galp-(1→6)-αβ-d-Galp, a branched pentasaccharide with a chain of α-d-Galp-(1→3)-α-l-Araf-(1→3)-β-d-Galp-(1→6)-αβ-d-Galp substituted at O-4 of Galp′ with α-l-Araf units, and a doubly branched heptasaccharide with a chain of α-d-Galp-(1→3)-α-l-Araf-(1→3)-β-d-Galp-(1→6)-αβ-d-Galp, disubstituted, respectively, at O-4 and O-6 of Galp′ with α-l-Araf and α-l-Rhap-(1→4)-β-d-GlcpA groups. A small amount of a related heptasaccharide was also present, with non-reducing end units of β-Arap, in place of those of α-Galp. These oligosaccharides were characterized by 1D and 2D NMR spectroscopy, ESI-MS, and in three cases, methylation analysis for final confirmation. The oligosaccharides should represent, with the probable exception of the galactosyl reducing ends, side-chain structures present in the gum arabic polysaccharide.