## 3 Structure of Chitosan  
Chitin is a partially crystalline mono-polymer and consists of more than 5000 \(\beta\)-(1,4)-linked \(N\)-acetyl-p-glucosamine with over 106 molecular weight (MW). Chitosan is a partially or totally deacetylated product of chitin comprising of \(\beta\)-(1\(\rightarrow\)4)-2-amino-b-glucose and \(\beta\)-(1\(\rightarrow\)4)-2-acetamido-d-glucose units with the latter typically above 80% [18]. The basic components of chitosan are glucosamine and chitobiose and it owns a double helix structure comprising of six sugar residues that form a spiral plane with a pitch of 0.515 nm. The structure of chitosan is depicted schematically in Figure 1, where \"n\" specifies the degree of polymerization of glucosamine and \(N\)-acetylglucosamine units.  
The secondary structure of chitosan is formed by a hydroxyl group, amino group and \(N\)-acetyl group on the molecular chain, which is involved in the formation of intra-molecular and inter-molecular hydrogen bonds of chitosan [19]. In the chair-like structure of chitosan glucosamine residues, there are two intramolecular hydrogen bonds, one of which forms C3-OH with the glycoside group on another adjacent chitosan molecular chain, and the other forms C3-OH with the oxygen atom on the adjacent chitosanfuran ring. The C2-NH2, C3-OH and C6-OH functional groups of chitosan can form intramolecular and intermolecular hydrogen bonds, so that it has the physicochemical properties of swelling, water retention and adsorption, and therefore is hard to be digested and absorbed [20].  
The single-chain is linear, every 10.1-10.5 A along the chain axis undergoes one full twist [21]. Linear aggregates and rigid crystalline domains are formed since abundant hydroxyl groups, highly reactive amino groups and \(N\)-acetyl counterparts have strong intramolecular and intermolecular hydrogen bonding tendencies. According to the different biological functions and natural sources, and the different structural forms of chitosan are differentiated by the different arrangements of carbohydrate chains [22]. Nuclear magnetic resonance spectroscopy (NMR) and X-ray model are helpful to clarify the three crystal types of chitosan: \(\alpha\), \(\beta\) and \(\gamma\) chitosan, each of which has diverse natural sources and biological functions.  
The \(\alpha\) conformation is the most common heteromorphic conformation, consisted of two reverse parallel chains, where the units are orthogonal, and has high thermodynamic stability due to the hydrogen interaction between the chains probably, usually separated from the exoskeleton of the crustacean, cell wall of yeast and cuticle of the arthropod. It is generally deposited with minerals to form a hard shell and compact structure. The \(\beta\)-conformation is the second common allomorph, composed of two parallel chains, where the units are monoclinic, and has weaker intermolecular forces perhaps owning to the polymer chains arranged in a parallel fashion, demonstrating a certain degree of hardness, flexibility and fluidity, usually combined with collagen, reside in squid cartilage [23]. The \(\gamma\)-form rarely has been seen and possesses two identical chains in association with one reverse and top-down chain, which seems to be a combination of \(\alpha\) and \(\beta\) forms rather than a different variant,existing in the thick epithelium of the squid stomach [24]. Chitin and chitosan both contain active hydroxyl and amino groups, however, chitin is usually more crystalline than chitosan, so chitosan may be more suitable for the preparation of reagents and biomaterials. The crystallinity of chitosan is affected by the degree of deacetylation. The completely deacetylated chitosan has the characteristics of a uniform molecular chain, good regularity and high crystallinity. As shown in Figure 2, the crystal structure of \(\alpha\)-chitosan, \(\beta\)-chitosan and \(\gamma\)-chitosan are respectively matched to anti-parallel, parallel and alternated arrangements of polymer chains.