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196
An interesting situation occurs in chitosans, where
the N-Acetyl-D-glucosamine (GlcNAc) residues
can be hydrolysed at rates comparable to glucose
and other ‘standard’ hexoses. In contrast, the
glucosamine (GlcN) residues are very resistant,
being hydrolysed about 1000 slower than GlcNAc.
The reason is the amino group, which in acid
becomes protonized:
-NH
2
+ H
+
→
-NH
3
+
(pK
a
= ca 6)
Thus, it becomes positively charged during the acid hydrolysis. The proximity
to the glycosidic oxygen largely reduces the protonation (becomes much less
basic) of the latter, thus reducing the rate of hydrolysis.
5.1.4.
Intramolecular acid hydrolysis
in alginates
Since H
+
catalyses the acid hydrolysis it is normally found that the rate of
hydrolysis is proportional to the concentration of H
+
:
k
∝
H
+
⎡⎣ ⎤⎦
⇔
log
k
= − −
log
H
+
⎡⎣ ⎤⎦
(
)
+
constant
⇔
log
k
= −
pH
+
constant
A drop in one pH unit therefore increases the rate of hydrolysis 10 times. A
plot of log k (rate constant) versus pH should ideally give a straight line with
slope of -1 for most glycosides.
In alginates and pectins deviations from this trend is observed in the pH range
1-5, where the rate of hydrolysis is slow, but larger than the trend extrapolated
from more acidic conditions. The figure below is reproduced from a classical
article written by Smidsrød and co-workers
39
. It shows data for alginate
compared to methyl cellulose (the latter behaving as predicted by the
equations above):
39
Note that
Δ
(1/[
η
]) is used as a measure for the rate of depolymerisation. This follows from
the Mark-Houwink equation: [
η
] = KM
a
and taking advantage of the fact that the exponent 1 is
close to 1 in this case. Thus 1/M
w
is proportional to 1/[
η
].
HOH
2
C
NH
3
HO
O
O
D-glucosamine (GlcN)