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5.1.6. Alkaline hydrolysis
As already mentioned, glycosides are generally stable under alkaline
conditions. Only in strong alkali and at high temperatures can alkaline
hydrolysis be observed in – for example – cellulose. However, elimination-
sensitive polysaccharides are very labile to alkali as described below.
5.1.7. Alkaline
β
-‐elimination
4-linked polyuronides such as alginates and
pectins (especially highly esterified pectins)
are labile in alkaline solutions. This lability is
linked to the carbonyl group at C6, which
make the sugars susceptible to alkaline
β
-
elimination. In alginates, the rate of
depolymerisation is proportional to the OH
-
concentration when pH is above 10.5. The
reaction involves the following steps:
1.
The proton at C5 is much more acidic
than other ring-protons because it is
located in the
α
-position relative to the
carbonyl group at C6. It reacts with OH
-
to produce a carbanion intermediate.
2.
The linkage between C4 (
β
-position)
and the adjacent sugar is cleaved,
giving rise to a 4,5-unsaturated sugar.
If the carboxyl group at C6 is esterified (no
negative charge) such as in pectins, the
β
-
elimination reaction is orders of magnitude faster than for the corresponding
un-esterified sugars.
5.1.8. Enzymatic degradation
a) Hydrolases
Most polysaccharide degrading enzymes work by applying an acid catalysed
degradation mechanism. In such cases at –COOH group (aspartic acid or
glutamic acid R-group) within the enzyme functions at proton donor. Other
Alkaline b-elimination of alginate
O
OH
HO
O
O
O
O
OH
HO
O
O
O
H
H
OH
O
OH
HO
O
O
O
O
OH
HO
O
O
O
H
O
OH
HO
O
O
O
O
OH
HO
O
O
O
H
O
OH
HO
O
O
O
OH
H
H
2
O