3.3 Chemistry of Chitin
35
While the insolubility of chitin poses a difficulty in processing it into other products, this characteristic of chitin however comes in use during the extraction process.
Extraction of chitin from crustacean shells such as crabs involves dissolving the
collagenous component of the shell in a strong alkali followed by dissolving the
minerals (mostly calcium carbonate) in acid. What’s left behind is the only component of these shells that does not dissolve in either acidic or alkali solvents, chitin.
Solubility of chitosan increases as the chain length decreases. Chitosan oligomers
are chitosan chain with short chain lengths. Degree of polymerization on chitosan
could be as low as 5. We could also have chitosan dimers and trimers. This shorter
chain allows for improved solubility which aids certain applications (Azuma et al.
2015).
The secondary and tertiary structures of polymers are dependent on the presence
of functional groups within the chain and the interactions between the functional
groups within the chain and surrounding chains. Chitin has within its chain acetyl,
hydroxyl and amino groups. Intermolecular bonding and intramolecular hydrogen
bonding between these groups cause chitin to fold in on itself and aggregate, making
it unready to form hydrogen bonds with the solvents, hence the insoluble nature of
chitin.
3.3.3 Isomers
Chitin exhibits polymorphisms in three different forms, α, β and γ. The α and β
forms are the more crystalline forms and have received more attention (Kalut 2008).
The chitin tertiary structure comprises of six strands of chitin polymer chain wound
together in a protein-like helical structure to form crystalline chitin microfibrils (Roy
et al. 2017). The α conformation is the most abundant in nature. In the α conformation,
the chitin polymer chains are arranged in alternating manner. In the β chitin, the
polymers are arranged in the same direction while in the γ conformation they are
randomly arranged.
3.3.4 Variation with Source
The distribution of the acetyl group within the chitin and chitosan structure is completely random. This poses a challenge in achieving a precise standard for solubility
and solution properties. Results from one research group may depend highly on the
batch being used. There is yet to be a standard correlation between parameters such
as degree of acetylation and species such that chitin derived from a particular species
would correlate with, for example, molecular weight, solubility and degree of acetylation. The solubility of chitin and chitosan is dependent on the primary, secondary,
tertiary and quaternary structure of the polymer; these are affected by the degree
of deacetylation which in turn varied from species to species and batch to batch.
35
While the insolubility of chitin poses a difficulty in processing it into other products, this characteristic of chitin however comes in use during the extraction process.
Extraction of chitin from crustacean shells such as crabs involves dissolving the
collagenous component of the shell in a strong alkali followed by dissolving the
minerals (mostly calcium carbonate) in acid. What’s left behind is the only component of these shells that does not dissolve in either acidic or alkali solvents, chitin.
Solubility of chitosan increases as the chain length decreases. Chitosan oligomers
are chitosan chain with short chain lengths. Degree of polymerization on chitosan
could be as low as 5. We could also have chitosan dimers and trimers. This shorter
chain allows for improved solubility which aids certain applications (Azuma et al.
2015).
The secondary and tertiary structures of polymers are dependent on the presence
of functional groups within the chain and the interactions between the functional
groups within the chain and surrounding chains. Chitin has within its chain acetyl,
hydroxyl and amino groups. Intermolecular bonding and intramolecular hydrogen
bonding between these groups cause chitin to fold in on itself and aggregate, making
it unready to form hydrogen bonds with the solvents, hence the insoluble nature of
chitin.
3.3.3 Isomers
Chitin exhibits polymorphisms in three different forms, α, β and γ. The α and β
forms are the more crystalline forms and have received more attention (Kalut 2008).
The chitin tertiary structure comprises of six strands of chitin polymer chain wound
together in a protein-like helical structure to form crystalline chitin microfibrils (Roy
et al. 2017). The α conformation is the most abundant in nature. In the α conformation,
the chitin polymer chains are arranged in alternating manner. In the β chitin, the
polymers are arranged in the same direction while in the γ conformation they are
randomly arranged.
3.3.4 Variation with Source
The distribution of the acetyl group within the chitin and chitosan structure is completely random. This poses a challenge in achieving a precise standard for solubility
and solution properties. Results from one research group may depend highly on the
batch being used. There is yet to be a standard correlation between parameters such
as degree of acetylation and species such that chitin derived from a particular species
would correlate with, for example, molecular weight, solubility and degree of acetylation. The solubility of chitin and chitosan is dependent on the primary, secondary,
tertiary and quaternary structure of the polymer; these are affected by the degree
of deacetylation which in turn varied from species to species and batch to batch.