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1.3.3.
From
chitin
to
chitosan: Chemical de-‐N-‐acetylation
Chitosans are formed by partial de-N-acetylation of chitin, where the amide
linkage is hydrolyzed in alkali, converting GlcNAc to D-glucosamine (GlcN =
D). Glucosamine differs from glucose only at C2, having an amino group (-
NH
2
) instead of a hydroxyl. Hence, chitosans also resemble cellulose, but has
widely different properties just because of the amino group.
1.3.4. Chain geometry
Chitosans (and chitin) have also the same backbone geometry as both
cellulose and mannuronan: All sugars are
4
C
1
, and the
β
-1,4 linkages are
diequatorial.
Figure 31. Structure of chitosan indicating ring conformation and linkage geometries
Note also that each residue is rotated about 180 degrees compared to
neighbouring residues. Again, this is based on the chemistry of cellulose in
the crystalline phase, as well as molecular modelling. The latter excludes
almost all other possibilities.
O
CH
2
OH
OH
NH
2
O
O
CH
2
OH
OH
NH
O
O
CH
2
OH
OH
NH
2
O
O
CH
2
OH
OH
NH
2
O
O
CH
3
NH
3
+
O
HOH
2
C
NH
3
+
HO
O
O
HOH
2
C
HO
O
NH
3
+
O
HOH
2
C
NH
3
+
HO
O
O
HOH
2
C
HO
Figure 30. Chitosan structure: Some of the N-acetyl
groups have been removed by alkaline hydrolysis.
Residual acetyl groups (GlcNac) defines F
A
.