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51
1.3.5.
F
A
: The
fraction of A
(GlcNAc)
residues
The type of chitosan is commonly described in terms of the degree of de-N-
acetylation. Typically, commercial chitosans are 80-90% de-N-acetylated,
resulting in 80-90% GlcN and 10-20% (unreacted) GlcNAc. It is, however,
customary and fundamentally more appropriate to describe the composition of
polymers in terms of their composition rather than what has been removed.
Thus, the fundamental parameter is the fraction of N-acetylated residues, F
A
:
ž
F
A
=
fraction of GlcNAc =
n
GlcNac
n
GlcNAc
+
n
GlcN
=
1
−
F
D
Note analogy to F
G
in alginates. The use of fractions is also beneficial for
considering sequences. In chitosans the remaining A units seem to be
randomly distributed. The content and length of various block types
(..AAAAA…, ..DDDDDD…, ADADAD…) are thus found by simple statistical
rules (Bernoullian statistics, which also applies to games like Yatzy and Lotto).
It is sufficient to know F
A
for a complete description:
F
AA
= F
A
2
, F
AAA
= F
A
3
, F
ADA
= F
A
F
D
F
A
= F
A
2
(1-F
A
) etc..
Commercial chitosans have F
A
between 0.10 and 0.20, but the whole range of
F
A
from 0.001 to about 0.70 is available in some cases due to novel (industrial
scale) de-N-acetylation processes. The molecular weight (M
w
) of commercial
chitosans is typically 3-500.000 g/mol (Da). Lower molecular molecular
weights are easily produced by chemical or enzymatic degradation. Recently,
chitosan oligomers (DP 10-50) have attracted attention in some biomedical
applications.
1.3.6. Polyelectrolyte properties
Because of the –NH
2
(amino) group chitosans are bases, involving the
equilibrium:
-NH
3
+
= -NH
2
+ H
+
pK
a
= ca. 6.5 (for F
A
= 0.01)
Thus, when pH = pK
a
the 50% of the GlcN residues are positively charged
16
.
At pH 7.4 (physiological pH) chitosans still have some positive charges,
accounting for their ability to bind to DNA (negatively charged at pH 7.4).
16 For a full and general overview of the relationship between charges, pH and pKa, see textbook.