Biomedical Engineering Reference
In-Depth Information
i. h e hydroxyl groups in the polysaccharide unit can react
with HO
by the typical H-abstraction reaction.
ii. HO
can react with the residual free amino groups NH
2
to
form stable macromolecule radicals.
iii. h e NH
2
groups can form ammonium groups NH
3
+
by
absorbing hydrion from the solution, then reacting with
HO
through addition reaction.
h e presence of more than two free amino groups in chitooligosaccha-
rides may be mandatory for the antioxidant activity because glucosamine
and N
-
acetylchitooligosaccharides were not ef ective at all in inhibition of
H
2
O
2
-induced hydroxylation of benzoate. On the other hand, the oligosac-
charide of glucosamine with DP 2-5 as chitobiose, chitotriose, etc., were
active and shown to possess antioxidant properties against carcinogen
induced oxygen radical species, as well as hydrogen peroxide released from
activated polymorphonuclear leukocytes [280, 281]. Among the chitosans
DDA 90, 75 and 50 %, the most potent antioxidant found was chitosan
DDA 90 % [282]. h is is due to accessibility of more number of free -NH
2
groups for reaction with radicals.
Variation in the MW of chitosan DDA 90 % inl uenced the antioxidant
activity. Medium MW chitosan MW 1-5 kDa, DDA 90 %, exhibited high-
est activity in comparison to high MW (10-5 kDa) and low MW (<1kDa)
chitosans of same and lesser DDA [283]. Other studies also reported pro-
nounced activity in low molecular weight chitosans [284 285]. Evaluation
of chito oligomers of varied MW (2.3, 3.27, 6.12, and 15.250 kDa) obtained
from high MW chitosan (850 kDa) also displayed the similar trends of anti-
oxidant activity and MW relation and lack of activity in initial high MW
chitosan [286]. A comparison of the antioxidant action of high MW chi-
tosans (604, 931 kDa) with that of low MW chitosans (2.8, 17.0, 33.5, 62.6,
87.7 kDa), showed that low MW chitosans were more ef ective in prevent-
ing the formation of carbonyl groups in human serum albumin exposed
to peroxyl radicals and scavenging nitrogen centered radicals [287]. h e
plausible reason for such observation is short chain composition of low
MW chitosans or oligomers as compared to high MW chitosan. In short
chains, the ability to form intramolecular hydrogen bonds between N
2
-O
6
and O
3
-O
5
decline sharply, i.e., the hydroxyl and amino groups are acti-
vated and help the radical scavenging process. h e hydrolysates of chitosan
for the same reason were good antioxidants [287, 289].
Many of the chitosan derivatives abide by these observations. h e reported
antioxidant derivatives of chitosan are shown in Figure 6.3. With screening of
