Structure of arabinogalactan oligosaccharides derived from arabinogalactan-protein of Coffea arabica instant coffee powder.

Arabinogalactan-protein, previously isolated from instant coffee powder of Coffea arabica, has been subjected to partial mild acidic and enzymatic hydrolyses. Separation of obtained mixtures by size exclusion and HPLC chromatographies afforded series of oligosaccharides, structure of which were studied by NMR spectroscopy. Mild acidic hydrolysis afforded oligosaccharides without any αAraf substituent while after enzymatic hydrolysis αAraf was found in di-, tri-, and tetrasaccharides. In all cases αAraf was a terminal substituent linked separately to O3, O6, and to both, O3 and O6, of βGal residues. Identification of di-, tri-, and tetrasaccharides containing α-Araf enabled to distinguish in the (1)H NMR spectra αAraf signals linked to O6 and O3 of neighboring βGal unit. Composition of polymeric residues after enzymatic and mild acidic hydrolyses was also analyzed.