A high-viscosity glycoglucuronomannan from the gum exudate of Vochysia thyrsoidea: Comparison with those of other Vochysia spp.

The polysaccharide (VTh) extracted with water from the gum exudate of the trunk Vochysia thyrsoidea gave a solution of high viscosity and was homogeneous, with Mw 92.5 × 103 and dn/dc = 0.212. Partial hydrolysis gave a main chain of →4)-β-d-GlcpA-(1 → 2)-α-d-Manp-(1→ repeating groups. Since VTh contained 24% GlcA, neither its monosaccharide ratio (GC–MS of alditol acetates) nor methylation results were accurate. It was therefore carbodiimide-reduced to CR3VTh, which contained Ara, Xyl, Man, Gal, and Glc in a molar ratio of 28:4:29:19:25: methylation analysis showed mainly nonreducing end-units of Araf (25%), Arap (8%), and Galp (8%) and side-chain units of 3-O- (6%) and 3,4-di-O-subst. Galp (12%). Main-chain units of Manp were 2,3-di-O-substituted (17%), showing that they were substituted at O-3 by side chains, but there was less side-chain substitution of Glcp units, which were mainly 4-O-substituted (12%). Most Araf units in VTh and CR3VTh were single nonreducing end-units, and were present in groups of α-l-Araf-(1 → 3)-d-Manp and α-l-Araf-(1 → 3)-[α-l-Araf-(1 → 4)]-β-d-Galp-(1 → 3)-α-d-Manp. Other degradation products derived from VTh agreed with this structure. The degree of side-chain substitution of the same main chain was greatest with the gum exudate polysaccharide of V. tucanorum, and progressively less with those of V. thyrsoidea, and V. lehmannii. High-viscosity aqueous solutions were formed by the gum of V. thyrsoidea, in contrast with those of V. lehmannii and V. tucanorum.