Structural characterisation of hemicellulosic polysaccharides from Benincasa hispida using specific enzyme hydrolysis, ion exchange chromatography and MALDI-TOF mass spectroscopy

Abstract

Benincasa hispida fruit is an important source of water-soluble polysaccharides. Hemicellulosic polysaccharides extracted with 1- and 4-M KOH represented 120 mg/g of alcohol insoluble residues. The intrinsic viscosity of the extracted polymers (named B1OH and B4OH fractions) was 284 and 262 ml/g, respectively. The extracted polymers, which were further characterised by size exclusion chromatography, give one broad peak. The apparent molecular weight of the isolated polysaccharide was estimated to be around 130±40 kDa. Sugar composition analysis of both fractions demonstrates the presence of xyloglucan (XG) and xylan. Structural characterisation of hemicellulosic polysaccharides using specific enzyme hydrolysis, ion exchange chromatography (HPAEC) and Matrix Assisted Laser Desorption Ionisation-Time of Flight (MALDI-TOF) mass spectroscopy showed that B. hispida fruit XG is of XXXG type and contained XXXG, XXLG, XXFG and XLFG (named according to Fry et al. 1993) in the ratio of 31:28:41 as the major oligomeric subunits. This fucogalactoxyloglucan carries O-acetyl substituents exclusively on galactose-containing oligosaccharides subunits. Finally, enzyme hydrolysis together with HPAEC and MALDI-TOF-mass spectrometric analysis of the generated fragments shows that the xylans, also present in both fractions, exhibit a classical structure with a backbone of β-(1→4)-linked xylopyranosyl residues substituted with three 4- O-methyl glucuronic acid per 97 xylopyranosyl unit.