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.
