Oligosaccharides obtained by enzymatic hydrolysis of birch kraft pulp xylan: Analysis by capillary zone electrophoresis and mass spectrometry

Abstract

Neutral and acidic oligosaccharides were obtained from an unbleached birch kraft pulp by treatment with a Trichoderma reesei endoxylanase pI 9 and subsequently characterized using capillary zone electrophoresis (CZE) and matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS). The borate complexes of unsaturated acidic oligosaccharides having a 4-deoxy-β- l-threo-hex-4-enopyranosyluronic acid (4ΔUA) residue linked to a β- d-(1 → 4)-xylooligosaccharide backbone were separated by CZE and detected by their UV absorption at 232 nm without prior derivatization. Pre-column derivatization with the chromophore 6-aminoquinoline (6-AQ) followed by CZE in alkaline borate buffer using detection based on absorption at 245 nm was used in the case of neutral xylosaccharides. Furthermore, MALDI-TOF-MS was employed to determine the molecular masses of both unsaturated and saturated acidic oligosaccharides. The acidic oligosaccharides released upon endoxylanase treatment of the birch kraft pulp were a (4ΔUA)-β- d-xylotetraose, a (4ΔUA)-β- d-xylopentaose, a (4-O-methyl-α- d-glucurono)-β- d-xylotetraose and a (4-O-methyl-α- d-glucurono)-β- d-xylopentaose. Analysis after enzymatic hydrolysis with β-xylosidase and α-glucuronidase from Trichoderma reesei strongly indicated that the uronic acid residue in these acidic oligosaccharides was linked to the d-xylose unit adjacent to the non-reducing d-xylose unit. The neutral xylosaccharides obtained after endoxylanase treatment of the pulp sample were d-xylose, β-(1-4)- d-xylobiose and β-(1-4)- d-xylotriose.