Chitin

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Material type:	Polymer
Material group:	Homo polysaccharide, linear Chitin & derivatives
Monomer(s):	2-acetamido-2-deoxy-D-glucose
Links:	beta-(1-4)
Origin of the polymer:	Natural
CAS-Number:	1398-61-4
EINECS-Number:	215-744-3

Structure

(→4)-β-2-acetamido-2-deoxy-D-glucose(1→). Some degree of N-deacetylation may be found. Complete N-deacetylation is chitosan [1].

In chrystallography there are two polymorphic chitin forms: α and β: α-chitin, the most abundant of the two chitins. The helix repeats after 10.3 Å, analogous to the formation of cellulose. The α and β conformations depend on hydrogen bonds [4]. Crystallographic data in [2] and [5].

Chemical properties

Behavior in solution:
Chitin is tough, biodegradeable and relatively inert. It can not be dissolved in water and most ordinary solvents [1]. It can not readily be fabricated into fibers or membranes. Partially deacetylated chitin (chitosan) can be crosslinked by glutaraldehyde to an amber hydrogel [2].

Chitin is soluble in N,N-dimethylacetamide, N-methyl-2-pyrollidone and their mixed solvents in the presence of 5% LiCl [1,4]. Certain N,N-disubstituted amides with LiCl can dissolve chitin without degrading it. NMR and IR experiments suggests, that Li⁺ is coordinated to the carbonyl oxygen. For chitin the solubility parameter (δ) is 12.5, for the chitin-LiCl complex, d~11.2. Two good amide polymer solvents (a polymer solvent is a solvent with the approximately the same solubility parameter as the polymer) are dimethyl-acetamide (δ=10.8) and N-methyl pyrrolidinone (δ=11.3) [2].

Swelling of chitin by water: R_w/R_d: 1.2, S_w/S_d: 1.4, V_w/V_d: 1.7, (R_w: Diameter of fibers at wet state, R_d: Diameter of fibers at dry state, S_w: Cross section of fibers at wet state, S_d: Cross section of fibers at dry state, V_w: Volume of fibers at wet state, V_d: Volume of fibers at dry state) [2].


Chitin fibres
Chitin will coagulate, forming fibres, when dissolved in trichloroacetic acid (TCA) and a chlorinated hydrocarbon like chloromethane, dichloromethane or 1,1,2-trichloroethane at TCA concentrations 25-75% and chitin concentrations 1-10%. Fibres are formed extruding thru an acetone bath [1]. Another process for fibres is water/dichloroacetic acid with basic CuCO₃-NH₄OH as coagulant [1]. Several other methods on forming chitin fibres revolving around these two methods has been developed and patented. Alternatively a method involving N-Methyl-2-pyrrolidone, N,N-Dimethyacetamide and LiCl has been developed [1]. As TCA is a strong acid, chitin will degrade upon treatment. Generally chlorinated solvents are plastisizers of chitin. The tensile strength and elongations of the fibres is thus dependent on the process [1].


Deacetylation
Chitin can be deacetylated to chitosan by treatment with 40% NaOH at 120°C [1].


Solvents

Insoluble	Soluble
Acetone (coagulant) [1] Buthanol (coagulant) [1] Water [1]	Chloroalcohols in conjugation with aqueous solutions of mineral acids [1] Hexafluoroacetone [1] Hexafluoroisopropanol [1] N-Methyl-2-pyrrolidone + 5% LiCl [1] N,N-Dimethyacetamide + 5% LiCl [1] Trichloroacetic acid/chloral hydrate/methylene chloride (2:2:1) [1]

Misc.:
Chitin-fibers absorbs calcium ions very selectively, forming a chelate. The amide groups involvement is very clear, and the hydroxy groups (either the C(6) or the C(2)) has been suggested to participate too. The selectivity seems to be caused by the chelation mechanism [2].

Chitin, chitosan and the phosphorylates derivatives are known to adsorb uranium [2].

Chitin membranes are selective permeable, making them usable for separating water and solutes [2].

Chitin exhibits basic properties. Derivatives fall within two categories, in each case, the native N-acetyl groups are removed and the exposed amino function reacts either with acyl chlorides or anhydrides to give the group NHCOR or is modified by reductive amination to NHCH₂COOH [2].

Biological properties

Microbial activity:
Has a positice effect on the growth of certain bacterias [2]. Microcrystalline chitin stimulate the growth af certain bacterias [2]. Chitin in general stimulate human fibroblast proliferation at low concentrations and inhibit proliferation at higher concentrations [3].


Woundhealing:
Chitin has a positive effect on migration of fibroblasts [3].

Acetylations of 30% and 70% are effective at activating cytotoxic macrophages [2].

Chitin is degradeable by lysozyme. The absorbable degradation products are believed to underlie an observable enhancement in wound healing in vivo [3]. The degradation of chitin by lysozyme is not affected by substitutions at C(6) [2].


Misc.:
Chitin has been reported to be useful as a vehicle for sustained drug-release and dissolution of poorly soluble drugs, e.g. covalently bound herbicides [2].

Can be used as a feed additive for feeding chicks and hens [2].

Chitosan and chitin has been shown to increase the fecal excreation of Fe and lower the hemoglobin levels in rats at high dietary levels. Indications show, that chitin and chitosan cause no damage to the intestinal system, at dietary levels below 10% [2].

The adsorption of blood proteins decrease at the introduction of carboxy groups at C(6) in chitin [2].

Physical properties

Appearance
	Physical state @ 20°C:	Solid
	Color:	White [1]

Occurence, isolation & synthesis

Occurence
Abundant in fungal mycelia and as supporting material in crustaceans, insects etc. [1].

References

1: Kumar,M.N.V.R. A review of chitin and chitosan applications
React. Func. Polym. (2000) 46 1-27

2: Zikakis,J.P. Chitin, chitosan, and related enzymes
(1984) Academic press

3: Chung,L.Y., Schmidt,R.J., Hamlyn,P.F., Sagar,B.F., Andrews,A.M., Turner,T.D. Biocompatibility of Potential Wound Management Products: Fungal Mycelia as a Source of Chitin/chitosan and their Effect on the Proliferation of Human F1000 Fibroblasts in Culture
J. Biomed. Mater. Res. (1994) 28 463-469

4: Hirano,S., Horiuchi,K. Chitin Gels
Int. J. Biol. Macromol. (1989) 11 253-254

5: Sundararajan,P.R., Marchessault,R.H. Bibliography of crystal structures of polysaccharides 1977-1979
Adv. Carbohydr. Chem. Biochem. (1982) 40 381-399

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© Michael Pilgaard
Created: March 22, 2008