Characterisation of watersoluble polysaccharides produced during prehydrolysis of pinus radiata

An aqueous prehydrolysate (or prehydrolysis liquor) was produced during a mild hot-water prehydrolysis (90 minute ramp to 175 C) of commercial radiata pine wood chips. Oligosaccharide and polysaccharide material was separated from the concentrated prehydrolysate using solvent precipitation after most of the noncarbohydrate material was removed. These polymeric carbohydrates were fractionated based on charge and molecular weight by size-exclusion chromatography (SEC). The fractions were each analysed by a number of methods including MALDI-ToF mass spectrometry, and NMR. A number of different types of carbohydrate polymer structures were found that were produced due to the partial de-polymerisation of the wood hemicelluloses during the prehydrolysis process. The O-acetylated (galacto)glucomannans were the most extensively characterised. These partially-acetylated hexose-based polymers were the main type found and accounted for approximately 54% by mass of the polymeric carbohydrates. Most appeared to contained between 5 and 79 hexose units with differing degrees of acetylation. The average mol ratio of components in these polymers was calculated to be approximately 3.7 : 1.3 : 1 : 0.2 (D-mannosyl : acetyl : D-glucosyl : D-galactosyl). They had a structure consistent with a linear backbone of β-1,4- linked D-mannopyranosyl and β-1,4-linked D-glucopyranosyl units with acetyl groups attached at C-2 and C-3 positions of some D-mannopyranosyl units. The terminal D-galactopyranosyl units were likely to be attached at 1,4,6-linked Dmannopyranosyl branch points. Of the neutral (non-anionic) polysaccharides, this type was most prevalent in the higher molecular weight fractions. Anionic pentose-based polymers with a backbone of β-1,4-linked D-xylopyranosyl units were also characterised. Identified as (arabino)glucuronoxylans, they featured uronic acid groups consistent with 4-O-methyl-α-D-glucopyranosyluronic acids attached to the C-2 position of some D-xylopyranosyl units. Smaller amounts of terminal α-L-arabinofuranosyl units likely to be attached at β-1,3,4- linked D-xylopyranosyl branch points were also detected. These polymers appeared to mostly contain between 5 and 40 pentose units with between 1 and 4 uronic acid groups attached. The anionic fractions (approximately 30% by mass) also contained large amounts of D-galactopyranosyl and L-arabinosyl units along with some D-glucuronic and D-galacturonic acid residues. This suggested the presence of carbohydrates produced from the partial hydrolysis of arabinogalactans and pectins. The smaller molecular weight fractions of non-anionic polysaccharides were enriched in both 1,4-linked D-galactopyranosyl units and non-acetylated hexosebased polymers that contained between 5 and 30 hexose units; this suggested that significant amounts 1,4-galactan derived carbohydrates were present. Small amounts of oligomers containing only pentose units were detected in these smaller molecular weight fractions along with what appeared to be other uncharged fragments of the polysaccharide-types that were present in the anionic fractions