The Complex World of Polysaccharides
10
11. Parameters influencing the behavior of the biopolymer
The main parameters influencing the characteristics of chitosan are its degree of
deacetylation (DD) and molecular weight (Mw), which affect the solubility, rheological and
physical properties. Various grades of chitosan are available commercially, which differ
primarily in the degree of deacetylation and molecular weight. Different conditions such as
type and concentration of reagents, time and temperature employed throughout the
processing can affect the physical characteristics and performance of the final chitosan
product [50]. However, both DD and molecular weight can be further modified. For
example, DD can be lowered by reacetylation [51-55] and molecular weight can be lowered
by acidic or enzymatic depolymerisation [56-58].
12. Degree of Deacetylation (DD)
Deacetylation describes a reaction that removes an acetyl functional group. When the degree
of deacetylation of chitin reaches about 50% (depending on the origin of the polymer), it
becomes soluble in aqueous acidic media and is called chitosan. The solubilization occurs by
protonation of the –NH
2
function on the C-2 position of the D-glucosamine repeat unit,
whereby the polysaccharide is converted to a polyelectrolyte in acidic media. Chitosan is the
only pseudonatural cationic polymer and thus, it finds many applications that follow from
its unique character (flocculants for protein recovery, depollution, etc.). Being soluble in
aqueous solutions, it is largely used in different applications as solutions, gels, or films and
fibers.
Figure 4.
Chitin deacetylation
A highly deacetylated polymer has been used to explore methods of characterization [59].
The solution properties of a chitosan depend not only on its average DA but also on the
distribution of the acetyl groups along the main chain in addition of the molecular weight
[60-62]. Several methods have been proposed for alkaline deacetylation to obtain chitosan
[6,17]. The conditions used in the deacetylation determines the polymer molecular weight
and degree of deacetylation (DD).
Chitosan has been largely employed in many areas, such as photography, biotechnology,
cosmetics, food processing, biomedical products (artificial skin, wound dressing, contact
Is Chitosan a New Panacea? Areas of Application
11
lens, etc.), system of controlled liberation of medicines (capsules and microcapsules),
treatment of industrial effluents for removal of metallic and coloring ions. The amino
groups are responsible for the distinct characteristics attributed to this basic polymer
(compared to an acidic biopolymer). Therefore, the characterization of the polymer in
either chitin or chitosan is extremely important according to the structure-properties
relationship, defining a possible industrial application. Thus many techniques are available
to determine the degree of deacetylation. Elson Santiago de Alvarenga (2011) published on
line describing the most important parameters to be evaluated in chitosan as
“deacetylation degree” (DD) [63].
The methods for carrying out the analysis of the degree of deacetylation are: Elemental
analysis; Titration; HPLC; Infrared;
1
H nuclear magnetic resonance; CP-MAS
13
C NMR; CP-
MAS
15
N NMR; steric exclusion; nitrous acid deamination; thermal analysis.
13. Molecular weight
Another important characteristic to consider for these polymers is the molecular weight and
its distribution. The first difficulty encountered in this respect concerns the solubility of the
samples and dissociation of aggregates often present in polysaccharide solutions [16, 57, 64,
65, 66]. As to choice of a solvent for chitosan characterization, various systems have been
proposed, including an acid at a given concentration for protonation together with a salt to
screen the electrostatic interaction. The solvent is important also when molecular weight has
to be calculated from intrinsic viscosity using the Mark–Houwink relation.
14. Biological properties of chitosan
14.1. Biocompatibility
Biocompatibility of a biomaterial refers to the extent to which the material does not have
toxic or injurious effects on biological systems [67, 68]. One of the present trends in
biomedical research requires materials that are derived from nature as natural materials
have been shown to exhibit better biocompatibility with humans and because chitosan’s
monomeric unit, N-acetylglucosamine, occurs in hyaluronic acid, an extracellular
macromolecule that is important in wound repair. Additionally, the N-
acetylglucosamine moiety in chitosan is structurally similar to glycosaminoglycans
(GAGs), heparin, chondroitin sulphate and hyaluronic acid in which they are
biocompatible, and hold the specific interactions with various growth factors, receptors
and adhesion proteins besides being the biologically important mucopolysaccharides
and in all mammals. Therefore, the analogous structure in chitosan may also exert
similar bioactivity and biocompatibility [69, 70].
The potential of chitosan stems from its cationic nature and high charge density in solution.
An effective approach for developing a clinically applicable chitosan is to modify the surface
of the material that already has excellent biofunctionality and bulk properties [71]. Altering