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What is chitosan?

Chitosan is a polysaccharide that originates from chitin, one of the most abundant natural polymers in nature. Chitin, for its part, is made up of D-acetylglucosamine monomers, linked by β1-4 bonds, being discovered in 1811 by Bracconot.

Chitin is found in the shell of crustaceans (crabs, lobsters, etc.), insects, arachnids, on the wall of some fungi, in green algae and protozoa. In 1859, Rouguet treated chitin with potassium hydroxide, from which he obtained a polysaccharide that Hoppe-Seller later called chitosan.

Sodium or potassium hydroxide causes deacetylation of quinine, transforming D-acetylglucosamine into D-glucosamine: the other monomer that makes up chitosan, which is 60 to 100% deacetylated. Therefore, chitosan contains more D-glucosamine than D-acetylglucosamine.

One of the main differences between chitin and chitosan, in structural terms and in terms of intermolecular interactions, is that the polysaccharide chains of chitosan are held together more strongly than those of chitin; This is precisely due to the presence of acetyl groups.

Chitosan also has a slight positive charge due to the amine present in D-glucosamine, which allows it to interact with negatively charged surfaces, such as mucosal membranes. This property allows chitosan to be used to transport substances through membranes.

Chitosan also has numerous other uses, such as serving as a fungicide, wine protector, limiting bleeding, purifying water, etc.

Chitosan structure

Chitosan is a linear polysaccharide with a structure similar to that of cellulose. However, one of the hydroxyl groups of glucose is substituted by an amino group (-NH 2 ) or an acetylamino group (-HNOCH 3 ). Chitosan is made up of deacetylated units (orange) and acetylated units (blue), distributed randomly (see upper image).

These units are joined by β-1-4 bonds. Deacetylated units are made up of D-glucosamine molecules, while acetylated units are made up of D-acetylglucosamine molecules. Chitosan has a degree of deacetylation between 60 and 100%.

This indicates that D-glucosamine is found in a higher proportion than D-acetylglucosamine. Then, it can be considered that chitosan is a polysaccharide formed mainly by deacetylated units, constituted by D-glucosamine molecules linked by β-1-4 bonds.

Chitosan properties

Chitin is extracted from the shells of crustaceans, such as shrimp, which in turn serves as raw material for the industrial production of chitosan. Source: EHRENBERG Kommunikation, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons

Molecular weight

Its molecular weight is between 3 × 10 5 and 1 × 10 6 g / mol, depending on the chitin source from which the chittane is obtained. That is, its polysaccharide chains are large.

Elemental chemical composition

-Carbon: 44.11%

-Hydrogen: 6.84%

-Nitrogen: 7.97%

As can be seen, it is a considerably nitrogenous polysaccharide.

Physical appearance

It comes in the form of white or creamy white flakes or ground powder. It also appears as a white or off-white translucent sheet.

Taste and smell

Chitosan is odorless and tasteless.

Melting point

102.5 ºC

Density

1 g / cm 3

Solubility

It is insoluble in water and alkali. It is highly soluble in most dilute acids (pH <6.5), including formic acid, acetic acid, and hydrochloric acid.

Stability

It is stable at room temperature, and under the protection of nitrogen gas, it can withstand a temperature of 250 ºC without experiencing decomposition. It is incompatible with strong oxidizing agents.

Electric charge

The amino group of chitosan has a pKa of 6.5. This gives chitosan a slight positive charge and relative solubility in an acidic or neutral medium. Chitosan is a bioadhesive that can bind to negatively charged surfaces, such as epithelial cells.

Chitosan is a polycation that can form complexes with negatively charged molecules, such as lipids, proteins, and nucleic acids, modifying their behavior.

Chemical reactivity

The presence of hydroxyl groups (OH) and amines (NH 2 ) allows chitosan to form covalent bonds through etherification, esterification, and reductive amination reactions.

Biocompatibility and biodegradability

Chitosan is a positively charged non-toxic compound that can interact with the negative charges of the plasma membranes of different tissues, without causing damage to them. Hence the fact of the biocompatibility of chitosan.

Furthermore, chitosan is biodegradable by enzymatic action. For example, lysozyme is an enzyme that works by breaking β-1-4 bonds.

Chitosan Uses / Applications

Objects made of chitosan. Source: Jgfermart, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Water treatment

Chitosan is involved in water purification in several ways. It can sequester metal ions such as copper, lead, mercury, etc. It can also help remove food debris, dyes, and other negatively charged solids.

In addition, chitosan can bind suspended particles, increasing their size and allowing them to be removed in sediments. This produces an almost total removal of turbidity from the water.

farming

Chitosan is used in the coating of seeds and leaves, in the improvement of the response to drugs and / or long-acting fertilizers, in the improvement of germination and rooting, in the growth of the leaves and in the yield of the seeds.

In addition, chitosan has antifungal action that gives protection to plants . There is evidence that chitosan can act on the membranes and chromatin of plants, inducing changes that allow better growth.

Food protection

Chitosan is used to form an edible film that covers food, thus preventing contamination. Also, chitosan has activity against fungi and bacteria.

The addition of compounds to chitosan such as clove oil / β-cyclodextrin, enhances its antibacterial capacity, which allows it to act on bacteria such as: S. aureus, S. epidermis, E. coli and others.

Nanofiber formation

The electrospinning method uses an electrical force to stretch polymeric structures such as chitosan, producing fibers 50 to 500 nanometers in diameter. These fibers are biocompatible and biodegradable and are used as hemostatic and wound healing materials.

Medical uses

The list of uses and applications of chitosan in medicine is extensive, and the following can be named: orthopedic implants, tissue engineering, wound healing, internalization of drugs into tissues, creation of artificial skin, surgical sutures and controlled drug release.

In addition to the named medical uses, chitosan is used in bandages, sponges, burns covering, as an anti-inflammatory agent, in inhibiting the formation of dental plaque, in accelerating wound healing, and as a hemostatic agent.

Chitosan has been used in war conflicts to reduce bleeding. Its positive charge allows it to coat negatively charged erythrocytes, forming a complex that activates platelets that initiate the clotting process.

Drug internalization

Chitosan is also used for the transport and internalization of drugs into tissues. In the case of DNA that codes for a certain antigen, due to its negative charge it cannot interact with the plasma membrane.

This difficulty is eliminated by joining the positively charged chitosan with the negatively charged DNA, allowing the chitosan-DNA complex to be internalized to the cells responsible for immunity.

This mechanism, for example, could be used for the production of a vaccine against DNA that encodes a protein from the Covid 19 virus.

Clarification of wine and beer

Chitosan is added in the final stage of brewing to improve flocculation and the elimination of yeast cells, which allows for the clarification of the beer.

Chitosan, together with bentonite, gelatin and other substances, increase the rate of sedimentation of fruit remains and detritus that cause the turbidity of the wine. In addition, it reduces the presence of oxidized polyphenols.

Advantages and disadvantages

The incorporation of drugs into tissues through the intervention of chitosan has been studied. Likewise, it has been experimented with the incorporation of the DNA that codes for a given antigen into the cells responsible for immunity, using chitosan in the transfection process.

DNA is incorporated into cells in a limited way due to its negative charge.

This limitation can be solved by its binding to chitosan: a polycation that neutralizes the negative charge of DNA, and therefore, facilitates its incorporation into cells. This in order to produce an immune response against DNA.

This strategy could be used for the formation of vaccines against DNA that codes against an antigen present, such as a virus or other biological material that it is desired to control.

Advantage

The use of chitosan in the incorporation of genetic material (DNA) into cells (transfection) has the advantage that chitosan is a non-toxic, biocompatible and biodegradable material.

It would function as a transporter for DNA. In a subsequent stage, the chitosan-DNA complex can form parts of the vacuoles, which are formed by internalization into the cytoplasm of portions of the plasma membrane, in a process known as endocytosis.

Chitosan is a non-toxic material per se. It is digestible and has bactericidal action.

Disadvantages

Chitosan-mediated transfection depends, in vitro, on the degree of deacetylation of chitosan, its molecular weight, and the formation of complexes with DNA. This indicates that it is a complex process that depends on many factors.

Therefore, it is difficult to find the optimal conditions for the reproducibility of the process, which is a disadvantage.

Another example that can illustrate the advantages and disadvantages in the use of chitosan is the following: the use of chitosan to incorporate it into animal feed.

When the proportion of chitosan incorporated into food reaches 20% of the feed, cases of death of animals have been reported, probably due to an inhibition of the absorption of nutrients, which could be excreted in the feces coupled with chitosan.

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