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Structure of a glycoglucuronomannan from the gum exudate of Vochysia tucanorum (family Vochysiaceae)
- Wagner, R., Simas, F.F., Pereira, G.C.Z., Angeli, A., Brito, J.O., Woranovicz-Barreira, S.M., Delgobo, C.L., Sassaki, G.L., Iacomini, M., Gorin, P.A.J.
- Carbohydrate polymers 2007 v.69 no.3 pp. 512-521
- plant exudates, xylan, Vochysiaceae, mannans, trees, carbohydrate structure, oligosaccharides, chemotaxonomy
- 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 lead to 2)-α-d-Manp-(1lead to 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 lead to 3)-β-d-Galp-, α-l-Araf-(1lead to 3)-β-d-Galp-(1 lead to 3)-β-d-Galp-, α-l-Araf-(1 lead to 3)[α-l-Araf-(1 lead to 4]-β-d-Galp-, and α-l-Araf-(1 lead to 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 lead to 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.