Chitosan
Chitosan is a linear polysaccharide composed of randomly distributed β--linked D-glucosamine and N-acetyl-D-glucosamine. It is made by treating the chitin shells of shrimp and other crustaceans with an alkaline substance, such as sodium hydroxide.
Chitosan has a number of commercial and possible biomedical uses. It can be used in agriculture as a seed treatment and biopesticide, helping plants to fight off fungal infections. In winemaking, it can be used as a fining agent, also helping to prevent spoilage. In industry, it can be used in a self-healing polyurethane paint coating. In medicine, it is useful in bandages to reduce bleeding and as an antibacterial agent; it can also be used to help deliver drugs through the skin.
History
In 1799, British chemist Charles Hatchett experimented with decalcifying the shells of various crustaceans, finding that a soft, yellow and cartilage-like substance was left behind that we now know to be chitin. In 1859, French physiologist Charles Marie Benjamin Rouget found that boiling chitin in potassium hydroxide solution could deacetylate it to produce a substance that was soluble in dilute organic acids, that he called chitine modifiée. In 1894, German chemist Felix Hoppe-Seyler named the substance chitosan. From 1894 to 1930 there was a period of debate and confusion over the exact composition of chitin and particularly whether animal and fungal forms were the same chemicals. In 1930 the first chitosan films and fibres were patented but competition from petroleum-derived polymers limited their uptake. It was not until the 1970s that there was renewed interest in the compound, spurred partly by laws that prevented the dumping of untreated shellfish waste.Manufacture
Chitosan is produced commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans and cell walls of fungi. A common method for obtaining chitosan is the deacetylation of chitin using sodium hydroxide in excess as a reagent and water as a solvent. The reaction follows first-order kinetics though it occurs in two steps; the activation energy barrier for the first stage is estimated at 48.8 kJ·mol−1 at and is higher than the barrier to the second stage.The degree of deacetylation can be determined by NMR spectroscopy and the degree of deacetylation in commercially available chitosan ranges from 60 to 100%. On average, the molecular weight of commercially produced chitosan is 3800–20,000 daltons.
Nanofibrils have been made using chitin and chitosan.
Chemical modifications
Chitosan contains the following three functional groups: C2-NH2, C3-OH, and C6-OH. C3-OH has a large spatial site resistance and therefore is relatively difficult to modify. C2-NH2 is highly reactive for fine modifications and is the most common modifying group in chitosan. In chitosan, although amino groups are more prone to nucleophilic reactions than hydroxyl groups, both can react non-selectively with electrophilic reagents such as acids, chlorides, and haloalkanes to functionalize them. Since chitosan contains a variety of functional groups, it can be functionalized in different ways such as phosphorylation, thiolation, and quaternization to adapt it to specific purposes.Phosphorylated chitosan
Water-soluble phosphorylated chitosan can be obtained by the reaction of phosphorus pentoxide and chitosan under low-temperature conditions using methane sulfonic acid as the catalyst; phosphorylated chitosan with good antibacterial activity and ionic properties can be prepared by graft copolymerization of chitosan monophosphate.The good water solubility and metal chelating properties of phosphorylated chitosan and its derivatives make them widely used in tissue engineering, drug delivery carriers, tissue regeneration, and the food industry.
In tissue engineering, phosphorylated chitosan exhibits improved swelling and ionic conductivity. Although its crystallinity is reduced, its tensile strength remains largely unchanged. These properties make it useful for creating scaffolds that can support bone tissue regeneration by binding growth factors and promoting stem cell differentiation into bone-forming cells. Additionally, to enhance the solubility of chitosan-based hydrogels at neutral or alkaline pH, the derivative N-methylene phosphonic acid chitosan has been developed. This material maintains good mechanical strength and improve cell proliferation, making it valuable for biomedical applications.
Thiolated chitosan
chitosan is produced by attaching thiol groups to the amino groups of chitosan using a thiol-containing coupling agent. The primary site for this modification is the amino group at the 2nd position of chitosan's glucosamine units. During this process, thioglycolic acid and cysteine mediate the reaction, forming an amide bond between the thiol group and chitosan. At a pH below 5, thiol activity is reduced, which limits disulfide bond formation.The modified chitosan exhibits improved adhesive properties and stability due to the covalent attachment of the thiol groups. Lower pH reduces oxidation, enhancing its adhesion properties. Additionally, thiolated chitosan can interact with cell membrane receptors, improving membrane permeability and showing potential for applications in bacterial adhesion prevention, for example for coating stainless steel.
Ionic chitosan
There are two main methods of chitosan quaternization: direct quaternization and indirect quaternization.- The direct quaternization of chitosan amino acids treats chitosan with haloalkanes under alkaline conditions. Another method is the reaction of chitosan with aldehydes first, followed by reduction, and finally with haloalkanes to obtain quaternized chitosan.
- The indirect quaternization method refers to introducing small molecules containing quaternary ammonium groups into chitosan, such as glycidyl trimethyl ammonium chloride, trimethyl ammonium bromide, etc. Quaternary ammonium groups can further be introduced into the chitosan backbone via azide-alkyne cycloaddition, or by dissolving chitosan in alkali and urea and then reacting it with 3-chloro-2-hydroxypropyl trimethylammonium chloride, which provides a simple and green solution to achieve chitosan functionalization.
Properties
Solution
Unmodified chitosan is generally insoluble in pure water, but dissolves in dilute acidic solutions. It is insoluble in most organic solvents. This is because chitosan bahaves like a strong base, with its primary amine groups having a pKa of about 6.3 for the reaction. When enough hydrogen ions are present, the amine group becomes protonated, giving it a positive charge. This allows water molecules to better "pick up" chitosan in the form of a water-soluble cationic polyelectrolyte.Chitosan readily forms soluble salts with many organic acid anions, including formate, acetate, lactate, malate, citrate, glyoxylate, pyruvate, glycolate, and ascorbate. Chitosan can also be dissolved in aqueous, which is useful for minimizing excess acidity.
The solubility and pKa of chitosan is affected by DD%. How the acetyl groups are distributed on the chain also matters. As a polyelectrolyte, the protonation behavior of chitosan is best described by Kachalsky’s equation.
Gel
When an aqueous solution of chitosan is exposed to a basic environment, precipitation occurs to form a gel, specificall an anionic hydrocolloid. However, this "gel" is mechanically weak because there are not a lot of interactions between the chains. Chemicals can be added to encourge ionic, electrostatic, and hydrogen-bonding interactions between chains, making the gel tougher.The free amine groups on chitosan chains can make crosslinked polymeric networks with dicarboxylic acids to improve chitosan's mechanical properties.
Noncovalent interactions
As mentioned above, aqueous chitosan has many positively charged amine groups. This makes it readily bind to negatively charged surfaces such as mucosal membranes.Chitosan can also effectively bind to other surface via hydrophobic interaction and/or cation-π interaction in aqueous solution.
Biological properties
Chitosan is biodegradable and biocompatible.Chitosan enhances the transport of polar drugs across epithelial surfaces. The enhanced chitosan uptake is mainly due to the interaction of positively charged chitosan with cell membranes, activation of chlorine–bicarbonate exchange channels, and reorganization of proteins associated with epithelial tight junctions, thus opening epithelial tight junctions. However, it is not approved by the FDA for drug delivery. Purified quantities of chitosan are available for biomedical applications.
Chitosan inhibits the growth of different bacteria and fungi by mechanisms involving several factors, including the degree of deacetylation, pH, divalent cations, and solvent type.
Uses
Agricultural and horticultural use
The agricultural and horticultural uses for chitosan, primarily for plant defense and yield increase, are based on how this glucosamine polymer influences the biochemistry and molecular biology of the plant cell. The cellular targets are the plasma membrane and nuclear chromatin. Subsequent changes occur in cell membranes, chromatin, DNA, calcium, MAP kinase, oxidative burst, reactive oxygen species, callose pathogenesis-related genes, and phytoalexins.Chitosan was first registered as an active ingredient in 1986.