From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 25

(COO - ) 1.0 Na +

O

O

OH

* *

n
2.5 H 2 O NHSO 3 - Na +

Figure 1.19
Chitosan heparin analogues.

The heparin alternative can be formed by the following steps: dissolving 3 g of CrO 3 in
3 mL of water and 30 mL of acetic acid, adding the solution to a suspension of chitosan
perchloride during fast stirring, reacting at 25°C for 30 min, adding CrO 3 solution of equal
amounts in the reactant, stirring for 30 min, adding 2 g of CrO 3 , stirring for 1 h, decompos-
ing excess CrO 3 by using 25 mL of methanol after reaction, stirring for 15 min, keeping it
still, removing the supernatant carefully, filtering, washing deposits with methanol
until the methanol is colorless and acid free, washing deposits with ether, drying them in
vacuum to form 9.7 g of chitosan perchloride whose primary hydroxyls are oxidized into
carboxyls, dissolving the product in pyridine, treating the solution with chlorosulfonic
acid, and neutralizing by using 1 mol/L NaOH solution to transform the perchloride on
aminos into sulfate and make carboxyls react with Na. The product is quite like heparin in
infrared spectrometry (Figure 1.19).

1.5.6 Chelation

Each residue of chitin and chitosan has an acetyl or an amino on C2 and a hydroxyl on C3.
They are flat bonds so that they can chelate metal ions with certain ion radii at certain pH
values. Chitosan is able to chelate more metal ions.
Muzzarelli [80] points out that chitosan combines metal ions by ion exchange, absorption,
and chelation. Chitosan will not change in structure but in property due to chelation.
Usually, its color changes after chelation, such as red chelate formed by chitosan and tita-
nium ion, orange chelate formed by chitosan and metavanadate, green chelate formed by
chitosan and trivalent chromium, orange chelate formed by chitosan and hexavalent chro-
mium, yellowish-brown chelate formed by chitosan and ferrous, yellowish-green chelate
formed by chitosan and ferric, pink chelate formed by chitosan and cobalt ion, green chelate
formed by chitosan and nickel ion, and blue chelate formed by chitosan and copper ion.
Chitosan cannot chelate alkali metals or alkaline earth metals; therefore, chitosan may
separate transition metal ions from a solution of alkali metal or alkaline earth metal by
chelating as shown in Table 1.2. The most convincing example is as follows: after treating

TAble 1.2
Coexistence of Alkaline Metal Ions and Transition Metal Ions

Accumulation Rate (%)

Ions 1 h 2 h 8 h 18 h 24 h

Cr 3+ 33 71 91 94

Cr 3+ + 10% NaCl 35 42 74 93 95