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5.1. DEGRADATION OF POLYSACCHARIDES: CHEMISTRY
Polysaccharides can de depolymerised (fragmented) as can any polymer
(synthetic or biological). Different reaction mechanisms may be involved:
chemical, physical or biological. One example is the biodegradation of
cellulose by certain fungi (e.g.
Trichoderma reseei
) which produce cellulases
– enzymes which degrade cellulose.
Endo
cellulases break the long cellulose
chains into smaller fragments, and
exo
cellulases then cleaves off cellobiose
(Glc-Glc) from the chain termini. Other enzymes cleave the cellobiose
molecules to obtain glucose, which is further metabolized by the cells.
Complete cleavage of all linkages in a polysaccharide is required to identify
and quantify the component sugars. This is typically achieved by total
hydrolysis in 2N trifluoroacetic acid (strong, volatile acid) at 120
°
C for 2-4
hours. The liberated monosaccharides are easily identified and quantified by
GC-MS (after derivatization) or HPLC methods. The major challenge is the
presence of very resistant linkages and side reactions when labile sugars are
involved.
Industrial polysaccharides such as alginates are sometimes subjected to mild
(partial) hydrolysis to reduce the molecular weight from 3-400.000 Da to 30-
40.000 Da because this molecular weight range is optimal in certain
applications. In other cases it is essential to prevent degradation to preserve
the highest possible molecular weight (example: special cellulose).
In all cases, it becomes important to know the detailed mechanism of
degradation as well as the rate of cleavage of glycosidic bonds.
5.1.1. Glycosidic
linkages
Most glycosidic linkages are chemically speaking acetals
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, which are formed
by the reaction between a hemiacetal (the sugar ring) and an alcohol. The
figure illustrates hemiacetal formation in D-glucose, and subsequent acetal
formation by reaction with an –OH in a second D-glucose residue:
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For a keto sugar (e.g. fructose) the linkage will be a ketal