Structural investigation of hemicellulosic polysaccharides from Argania spinosa: characterisation of a novel xyloglucan motif

Abstract

Hemicellulose polymers were isolated from Argania spinosa leaf cell walls by sequential extractions with alkali. The structure of the two main polymers, xylan and xyloglucan, was investigated by enzyme degradation with specific endoglycosidases followed by analysis of the resulting fragments by high performance anion exchange chromatography (HPAEC) and matrix-assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF MS). The results show that A. spinosa xylan is composed of a β-(1 → 4)-linked- d-xylopyranose backbone substituted with 4- O-methyl- d-glucuronic acid residues. Xyloglucan oligosaccharide subunits were generated by treatment with an endo-(1 → 4)-β- d-glucanase of the xyloglucan-rich hemicellulosic fractions. MALDI-TOF mass spectra and HPAE-PAD chromatography of the pool of endoglucanase-generated xyloglucan oligomers indicated that A. spinosa cell wall contains a XXXG-type xyloglucan. In addition to XXXG, XXFG, XLXG/XXLG, XLFG fragments previously characterised in various plants, a second group of XXXG-type fragments was detected. The primary structure of the major subunit was determined by a combination of sugar analysis, methylation analysis, post-source decay (PSD) fragment analysis of MALDI-TOF MS and 1H NMR spectroscopy. This fragment, termed XUFG, contains a novel β- d-Xyl p-(1 → 2)-α- d-Xyl p side chain linked to C-6 of the second glucose unit from the nonreducing end of the cellotetraose sequence.