Structure of the fucose-containing acidic heteroxylan from the gum exudate of Syagrus romanzoffiana (Queen palm)

The gum exudate of S. romanzoffiana contained a heteropolysaccharide (SRP), which was isolated in 80% yield, contained Fuc, Ara, Xyl, Gal, Glc and uronic acid in a molar ratio of 23:25:41:1:3:8, and had [α]D25−72°. It was homogeneous with Mw 1.4×105 and Mw/Mn∼1.0 and formed high viscosity, aqueous solutions. Carboxy-reduction provided neutral material (SRP-CR), having glucose and its 4-O-methyl derivative in a molar ratio of ∼3:1. Methylation analysis of SRP showed a highly branched structure with non-reducing end-units of Araf (16%), Fucp (10%) and Xylp (12%), 3-O- (5%), 2-O- (17%), 3,4- (7%) and 2,4-di-O-substituted Araf (10%), and 2,3,4-tri-O-susbtituted Xylp units (21%). The 13C-NMR spectrum of SRP was complex and contained at least 8 C-1 signals. The 13C-NMR spectrum and methylation analysis of Smith-degraded polysaccharide (SRP-SM) were consistent with a (1→4)-linked β-Xylp main chain. Partial hydrolysis of SRP gave rise to α-GlcpA-(1→2)-β-[Xylp-(1→4)]0-1-αβ-Xylp and 4-Me-α-GlcpA-(1→2)-αβ-Xylp. Milder hydrolysis conditions gave rise a mixture of oligosaccharides that were fractionated by charcoal-Celite column chromatography. The 10% aq. EtOH fraction (OL-10) gave a main GC spot with RGal 1.2 and minor ones with RGal 1.07 and 0.78, and was characterized as a mixture of β-Xylp-(1→2)-αβ-Xyl, Fucp-(1→4)-αβ-Xylp, and Fucp-(1→3)-αβ-Ara. Small amounts of fucobiose were also present.