Chitin is the second most naturally abundant nitrogen-containing polysaccharide on earth (Dutta et al., 2004). A polysaccharide is a natural ubiquitous polymer whose molecules consist of simple sugars (monosaccharides/monomers) bonded together by specific types of covalent bonds known as glycosidic linkages (Udayan et al., 2017). 

The modified polysaccharide is composed of 2-(acetylamino)-2-deoxy-D-glucose (N-acetyl-D-glucosamine) units which form β-(1→4)-linkages, similarly to those formed between glucose subunits in cellulose (Dutta et al., 2002). Although it is not found in cellulose-containing organisms, chitin is often viewed as a cellulose derivative as a result of its structural similarity with cellulose. Chitin is in fact structurally identical to cellulose with the exception of an acetyl amine group (-NHCOCH3) replacing one hydroxyl group at the C-2 position of each monomer (Hudson & Smith, 1998). This modification allows for increased hydrogen bonding between neighbouring monomers, giving rise to a stronger, resilient chitin-polymer matrix (Dutta et al., 2004).

Chitin is insoluble in water, however once deacylated a soluble chitin-derivative arises (Cheung et al., 2015). Chitosan is the primary derivative of chitin recognised as a linear polymer composed of haphazardly dispersed N-acetyl-D-glucosamine and β-linked D-glucosamine. Chitosan, similarly to chitin, has various chemical properties most notably its available reactive amino and hydroxyl groups as well as its ability to chelate numerous transitional metal ions (Dutta et al., 2004).

Figure 1: Chemical structure of Chitin and Chitosan. Originated from Nilsen-Nygaard et al., 2015. 

Physical

Chitin is a polysaccharide that is found naturally occurring within a multitude of different organisms such as arthropods and fungi (Younes & Rinaudo, 2015). It is a colourless (Younes & Rinaudo, 2015), minimally toxic substance, that is not soluble in the solvents commonly available and is not reactive within the mammalian gastrointestinal system (Rinaudo, 2006). Moreover, chitin is a biodegradable molecule since the enzyme which breaks it down, known as chitinase, is abundant in nature (Rinaudo, 2006). These organisms synthesise chitin for use within their cell walls or exoskeletons and within these structures, it is found in its native conformation as organised, crystal-like microfibrils (Younes & Rinaudo, 2015). 

Chitin exists in two different polymorphic states (Tsurkan et al, 2021), which are known as allomorphs, and these are the α form and the β form (Younes & Rinaudo, 2015; Rinaudo, 2006). In both forms, the chains of chitin are considered to be arranged in sheets that are strongly adhered to each other by multiple intra-sheet hydrogen bonds however the difference between the two forms is that in the α form, inter-sheet hydrogen bonds are also present which are produced by the hydroxylmethyl groups that are present in the chains (Younes & Rinaudo, 2015; Rinaudo, 2006). Moreover, the α form has been determined to be the more naturally occurring form, as the β form occurs less in nature (Younes & Rinaudo, 2015). There was also the description of a third allomorph of chitin, known as the γ form, however there is still considerable debate on whether this is a separate form of chitin or part of one of the other forms (Rinaudo, 2006; Tsurkan et al, 2021).       

Within the organisms that synthesise chitin as well as generally in nature, chitin is not found completely on its own in a pure state (Tsurkan et al, 2021). In fact, this polysaccharide is able to form strong bonds with a variety of different molecules such as proteins, biominerals like calcium carbonate, lipids, polysaccharides and pigments and due to this reason, within nature, chitin arranges itself with these molecules on a nano scale to form chitin-pigments, chitin-mineral composite biomaterials and chitin-proteins (Tsurkan et al, 2021). The ability of chitin to form these latter structures as well as its inherent insolubility in most solvents has made the identification of chitin (Tsurkan et al, 2021), as well as the elucidation of its physical properties quite challenging and thus, certain information pertaining to its physical properties is still limited (Rinaudo, 2006). 

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Biological

Chitin is a large structural polysaccharide found in many different species such as invertebrates, fish, fungi cell walls, and insects. It can be considered to be the second most abundant substance within the biosphere, cellulose being the first. 

This carbohydrate was first identified in Fungi by Henri Braconnot in 1811 where he named it ‘fungine’, whilst another scientist Auguste Odier isolated the compound from beetle cuticle and named it ‘chitine’ derived from Greek meaning ‘covering’. 

The biological function of chitin changes according to the organism it is found in. For instance, in soft-bodied organisms, it is produced as a mechanical support. Insects and arthropods utilise chitin as a component of their exoskeleton, as well as being present within the body wall, gut lining and mouth parts of insects. Whereas in chitinous fungi, chitin is used as a replacement for cellulose within their cell wall. Due to the presence of this molecule within so many insect structures and fungi, chitin degrading enzymes can be used within certain fungicides and insecticides to eradicate pests. 

Chitin has many applications whether in the medical industry or not, due to its non-toxicity, biodegradability, and antimicrobial properties. An example of the use of this material in the medical field is for the use of surgical stitches, which degrade/dissolve over time, thus not requiring the removal process.