Structure of a glycoglucuronomannan from the gum exudate of Vochysia tucanorum (family Vochysiaceae)

The polysaccharide (VTP) from the gum exudate of Vochysia tucanorum gave aqueous solutions of low viscosity. VTP contained two components, which on freeze–thawing, gave rise to soluble (S-VTP) and precipitated (P-VTP) fractions, with Mw 350,000 and 520,000 g/mol, and dn/dc 0.134 and 0.139, respectively. Acid hydrolysis of S-VTP provided Ara, Xyl, Man, Gal, Glc in a 43:1:5:18:2 molar ratio (GC–MS) with 31% uronic acid, whereas the values for P-VTP were 51:1:5:16:2 and 25%, respectively. Methylation analysis of S-VTP revealed 11 neutral, partially O-methylated alditol acetates, mainly from nonreducing end- (31%) and 5-O-substituted Araf (9%), 3-O- (16%) and 3,4-di-O-substituted Galp (9%), and 2,3-di-O-substituted Manp units (10%): P-VTP was similar, containing the same structures, consistent with their almost identical 13C NMR spectra. Partial hydrolysis of P-VTP removed side chains giving rise to the main chain (PH4h) of -β-d-GlcpA-(1 → 2)-α-d-Manp-(1 → 4)- repeating groups. A controlled Smith degradation of S-VTP gave a polysaccharide (S1-SVTP), shown by methylation analysis to contain α-l-Araf-(1 → 3)-β-d-Galp-, α-l-Araf-(1 → 3)-β-d-Galp-(1 → 3)-β-d-Galp-, α-l-Araf-(1 → 3)[α-l-Araf-(1 → 4]-β-d-Galp-, and α-l-Araf-(1 → 2)-α-d-Manp- groups, with the Araf region probably containing some additional, oxidized 5-O-substituted L-Araf units. This agreed with the structure of a polysaccharide (PH30), formed by milder hydrolysis of P-VTP, sufficient to only remove Araf groups. For a more reliable characterization and quantification of S-VTP structures, it was subjected to two carbodiimide reductions and methylation analysis of the product (CR2-SVTP) showed the former GlcpA units to be 3,4-di-O-substituted. Minute amounts of a (1 → 4)-linked β-d-xylan were present in the gum, along with free reducing oligosaccharides. ESI-MS–MS, 13C NMR, and methylation analysis showed Ara-Gal-Man side-chain sequences similar to those of the glycoglucuronomannan. Mainly present was Ara4-Hex2, whose daughter ions were Ara3-Hex2, and Ara2. The oligosaccharide mixture contained nonreducing end-, 2-O- and 3-O-substituted Araf, internal 3-O- and 3,4-di-O-substituted internal Galp, and 2-O-substituted Manp reducing end-units. The glycoglucuronomannan of Vochysia lehmannii gum, although having the same main-chain, contained less side-chains.