Biomedical Engineering Reference
In-Depth Information
CH
2
OH
CH
2
OH
O
O
O
O
O
CHCH
2
N(CH
3
)
3
+
Cl
-
OH
+
H
2
C
OH
*
*
*
*
NH
NH
2
n
n
C
-
Cl
+
(CH
3
)
3
NCH
2
CH
2
OH
Figure 1.18
Chitosan hydroxypropyltrimethyl ammonium chloride.
There is another structure for chitosan quaternary ammonium salt: instead of directly
binding the quaternary ammonium group with C2, a low-molecular-weight quaternary
ammonium is bound to amino. For instance, as shown in Figure 1.18, chitosan hydroxy-
propyltrimethyl ammonium chloride is formed by mixing a water solution of glycidyl
trimethyl ammonium chloride and chitosan in the mol ratio 3:1 in isopropanol at 90°C for
8 h, followed by filtering, washing, and drying the product.
Chitosan amino has two H atoms and so the theoretical substitution degree can reach 200%.
In the said reaction, it is 127.71% and the product has good water solubility. In fact, the
N-acylation part has referred to such a quaternary ammonium salt. This method can be
applied for preparing a series of chitosan quaternary ammonium salts of the same structure.
1.5.5 Oxidation
Chitin and chitosan can be oxidized by oxidants whose mechanisms are complicated.
Different oxidants require different pH values and form different products by different
mechanisms. They may oxidize C6 hydroxyl into an aldehyde group or carboxyl, oxidize C3
hydroxyl into carbonyl (ketone synthesis), and eliminate partly aminos or acetaminos, even
damage the pyranoid ring and glucosidic bond. Common situations are described below.
Periodic acid degradation is frequently used in polysaccharide structure research.
Periodic acid selectively cracks dihydroxyl or trihydroxyl. Chitin does not have such a
structure, and so it cannot be oxidized by periodic acid. The amino on C2 of chitosan has
similar properties to hydroxyl and so each residue consumes one molecule of periodic acid
and releases 0.6-0.7 mol/L ammonia in oxidization when the pH is 4.1.
Hydrolysis of chitosan by dilute hydrochloric acid can be accelerated using a little
sodium nitrite due to oxidative deamination. Chitosan oligosaccharides of different molec-
ular weights can be obtained by controlling the amount of sodium nitrite. For example,
chitosan oligosaccharide of average polymerization degree 13 can be formed by dissolving
2 g of chitosan in 100 mL of 6% acetic acid, reacting with 85.5 mg of sodium nitrite at 20°C
for 30 min and adjusting the pH to 7.4 with NaOH solution.
Primary hydroxyls of chitin and chitosan can be oxidized to form chitin oxide and chitosan
oxide, which are polysaccharide acids. Sodium salts formed by acids are water soluble. The
most significant finding is that chitosan is turned into a compound similar to heparin in
structure after its aminos are sulfated and primary hydroxyls are oxidized [79]. Such a com-
pound is an alternative to heparin and is prepared as follows: dissolving 10 g of chitosan
acetate powder in 50 mL of water, adding 200 mL of acetic acid to the solution, distilling to
form syrup, repeating the steps until the product cannot be dissolved by acetic acid, adding
acetic acid to the syrup to 1 L, slowly adding 12 mL of 60% perchloric acid solution forcefully,
and stirring until needle-like deposits, namely chitosan perchlorides, are formed.
Search WWH ::
Custom Search