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195
5.1.3. Different
sugars are hydrolysed at very different
rates
Certain sugars are very sensitive to acid hydrolysis, and require only
(relatively) mild conditions for complete hydrolysis. Among the most sensitive
sugars are the sialic acids, which are found as terminal sugars in many
glycoproteins.
N-acetyl-neuraminic acid
NANA (N-acetyl-neuramic acid) can be selectively cleaved from adjacent
sugars by mild conditions (0.05 M H
2
SO
4
, 80
°
C, 60 min). Note NANA is a 2-
deoxy sugar, which are generally hydrolysed 500 to 1000 times faster than
the corresponding glucose derivative. Also, NANA is a keto sugar (convince
yourself by drawing the corresponding Fisher structure). Ketoses are more
sensitive to acids than the corresponding aldose.
3,6-anhydro-sugars, which are found in
carrageenans and agarose, are quite sensitive
to acids. Hence, these polysaccharides degrade
at low pH, in particular if the temperature is
increased. The rate constant for degradation at
pH 1 at 37
°
C is around 10
-3
h
-1
(Hjerde et al.
(1996), Carbohydr. Res., 288, 175-187). Hence,
carrageenans will degrade in the human
stomach, producing oligosaccharides.
6-deoxyhexoses such as L-rhamnose (left) or
L-fucose are hydrolysed 5-25 times more
rapidly than galactose and glucose. The latter
typically requires 1 M H
2
SO
4
, 6h, 100
°
C for
complete acid hydrolysis.
Uronic acids are known to be quite resistant
to acid hydrolysis. Conditions needed for
complete hydrolysis also lead to destruction
of the liberated uronic acids.
O
COOH
O
OH
H
N
HOH
2
C
R
H
3
C
O
OH H
HO H
O
O
OH
HO
HO
H
3
C
R
L-rhamnose (6-deoxy-L-
mannose)
O
HO
HO
OH
COOH
O
R
D-glucuronic acid (GlcA)
O
H
2
C
O
OH
H
O
R
R
3,6-anhydro-D-galactose (4
linked)