Structural studies of the arabinogalactans in Mangifera indica L. fruit exudate

The arabinogalactan of ‘Nam Dokmai’ mango (Mangifera indica L.) fruit exudate was studied to elucidate major structural characteristics in comparison with commercially available type-II-arabinogalactans from acacia (gum arabic) and larch wood. From the latter two, the purified polysaccharide was distinguished by its FTIR, 1H NMR, and 13C NMR spectra and its strikingly low polydispersity around a peak molecular weight of ∼22,000. As regards carbohydrate composition and FTIR fingerprint, it ranged between the two comparison samples. The molar ratio of its major components galactose, arabinose, rhamnose, and glucuronic acid was 84:9:4:4. In the FTIR spectra, absorbance maxima ascribed to β-galactans (1076–1070 cm−1) and arabinans (1043–1032 cm−1) always predominated, but their ratios differed. 2D NMR analyses of the purified mango fruit exudate polymer revealed 11 components, including the (1→3)-linked and the (1→6)-linked β-galactopyranosyl residues of type-II-arabinogalactans. α-Arabinofuranosyl moieties, α-l-rhamnopyranosyl-(1→4)–β-glucopyranosyluronic acid, and 4-O-methyl-β-glucopyranosyluronic acid were on terminal positions. Unlike the comparison samples and various Anacardiaceae tree exudate gums, this polysaccharide had internal (1→6)-linked β-galactopyranosyl residues in abundance and internal β-arabinofuranosyl units besides a remarkable number of different α-arabinofuranosyl moieties. Its higher sensitivity to partial acid hydrolysis, which left polymeric glucuronogalactan fragments (overall molar galactose/glucuronic acid ratio: 97:3) within a broad molecular-weight range, suggested greater accessibility to internal cleavage of this polymer. 2D NMR showed these fragments to contain β-glucopyranosyluronic acid and its 4-O-methyl derivative terminally, besides the aforesaid internal β-galactopyranosyl moieties. The still prevalent (1→6)-linked β-galactopyranosyl units and limited increase in terminal β-galactopyranosyl residues indicated longer β-1,6-galactan sequences compared to larch gum.