Structural characterization of a rhamnogalacturonan I-arabinan-type I arabinogalactan macromolecule from starfruit (Averrhoa carambola L.)

• A complex rhamnogalacturonan I (RGI) was found in the edible tropical starfruit.
• A branched arabinan and a type I arabinogalactan are attached to RGI backbone.
• The arabinans side chains were found in higher proportions than the arabinogalactans.

A structural characterization of polysaccharides obtained from edible tropical fruit named starfruit (Averrhoa carambola L.) was carried out. After fractionation by freeze–thaw and Fehling precipitation, a pectic polysaccharide was obtained. It was composed of rhamnose, arabinose, galactose and uronic acid in the 5.0:72.5:12.1:10.4 molar ratios, respectively. A combination of monosaccharide, GPC, methylation and NMR analysis and enzymatic hydrolysis with endo-β-(1 → 4)-D-galactanase showed the presence of a rhamnogalacturonan I to which a branched arabinan and a type I arabinogalactan are attached. The arabinan moiety was formed by (1 → 5)-linked α-L-Araf units in the backbone, branched only at O-3 by (1 → 2)- and (1 → 3)-linked α-L-Araf units, while the type I arabinogalactan was formed by (1 → 4)- and (1 → 4,6)-linked β-D-Galp units in the backbone with (1 → 5)-, (1 → 3,5)- and (1 → 3)-linked α-L-Araf units as side chains.