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CELLULOSE AND CHITIN: STRUCTURE AND FUNCTIONS
Prepared by: Mohd. Ikram Bin Jabilin
Personal use only. Not been reviewed.



1.0 Introduction

Carbohydrates or saccharides are one of the compounds that play an important role in living organism. There are various compounds that fall into these categories and two of them are the cellulose and chitin. These compounds are the example of structural polysaccharides.

According to Wertz et al. (2010), cellulose was initially found and named by Anselme Payen in early 1838. It the most abundant organic material on the earth (Yang et al. 2011). Chitin also is categorize as one of the most abundant organic material. A structural model of the atomic structure of a-chitin was first established by Carlström (1957) and Minke and Blackwell (1978) by X-ray techniques (Fabritius et al. 2011).

Generally the structural of the cellulose and chitin are quite similar except for the small different in the functional group. In cellulose, the carbon 2 contain hydroxyl group while in chitin, the hydroxyl group is replaced by acetamido group. This different affected the properties of the cellulose and also the stability.

Due to their characteristics, cellulose and chitin has been well studied and many products were innovated from the cellulose and chitin itself and also their derivatives. Because of that, this paper will discuss more about the structure of cellulose and chitin and also their function naturally and artificially.

2.0 Structure of Cellulose

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Figure 1: Haworth projection of the general molecular structure of cellulose.

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Figure 2: Molecular structure of cellulose showing the 4C1 chair conformation.

Cellulose (C6H10O5)n is linear polymer of D-anhydroglucopyranose units where the repeated monomers are connected through β-1,4-glycosidic bond linkage. It also known as β-1,4-D-glucan (polyglucose) (Wertz et al. 2010). The order in numbering the carbon was shown in the figure 1 and 2. To understand more the structure of the cellulose, it can be started with the structure of its monomer that is β-D-glucopyranose (Figure 3).

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Figure 3: β-D-glucopyranose

The basic unit of the carbohydrates is called monosaccharide, for example glucose and fructose. This basic unit can be group into two types that is aldoses and ketoses based on the presenting of aldehyde and ketone group in the structure. D-Glucose was categorized as aldoses because the aldehyde group is present at the carbon number 1 (Figure 4).

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Figure 4: D-glucose

The six carbon compound such as glucose is called hexoses. These hexoses have four asymmetric centers (chiral carbon) in their acyclic form which is from carbon 2 to carbon 5. By referring to the formula to count the stereoisomer (2^n), the four chiral carbons will have 16 hexoses stereoisomer while the 16 stereoisomer are a pairs of 8 enantiomers (Wertz et al. 2010).

Enantiomer is the stereoisomer that are mirror images with one another. Each pairs of enantiomers has a sign whether D or L. (Mathews et al. 2000). Glucose in the cellulose is the type of D-enantiomers that is D-glucose (Wertz et al. 2010).

The D-glucose will form a ring structure. Each of the rings in the monomers of the cellulose is a pyranose rings. It is categorized as pyranose because the D-glucose forms a cyclic rings with six-membered form. The formation involved the linkage of the aldehyde group and the oxygen at in the open chain D-glucose as in figure 5 (Wertz et al. 2010).

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Figure 5: Formation of D-glucose ring. Note the linkage between aldehye group and carbon number 5.

The ring form will make the carbon 1 as the anomeric center, thus produce two diasteroiomers, called anomers which was designated as α and β. For β-D-Glucose, the hydroxyl group at the anomeric centre is equatorial of the plane. The pyranose rings are in the 4C1 chair conformation, where the carbon 4 and carbon 1 out the reference plane containing the four other ring atoms (Wartz et al. 2010).

The β-D-Glucopyranose will link to each other through β-1,4-glycosidic linkage. This bond causes the crystal structure of the cellulose to form a twofold helical conformation (two monomers per turn of the helix) with the angle of 1800 between two adjacent units (figure 6). The β-characters of the glycosidic bond in the cellulose are the result of the linkage of the monomers to the carbon 1 of the other monomer, thus forming an equatorial linkage (Wartz et Al. 2010).

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Figure 6: Conformation model of the cellulose chain in most crystal structures.

The linkage of the monomers forming a straight chain polymer. By referring to the ring structure, each monomer has three hydroxyl group attached to carbon number two, three, and number 5 [as a hydroxymethyl group (CH2OH)]. This hydroxyl group is responsible in the aggregations of the molecules into crystal by forming a hydrogen bond between same or a neighbouring chain as shown in figure 7 (Wartz et al. 2010).

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Figure 7: Hydrogen bond between the same and neighbouring chains.

Treatment and source of the cellulose will affect the structure such as in the aspect of type of crystal arrangement and the degrees of polymerization (DPs). Generally there are two type of the crystalline arrangement of cellulose, named with cellulose I and cellulose II. Cellulose I is the typical structure, while cellulose II is form by treating with some chemical. In the aspect of DPs, the cotton can have cellulose chain up to 20 000. The cellulose powder (treated from cellulose pulp) has a DPs range from 100-300 (Wartz et al. 2010).

3.0 Structure of Chitin

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Figure 8: Haworth projection of the general molecular structure of chitin.

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Figure 9: Molecular structure of chitin showing the 4C1 chair conformation.

Chitin (C8H13O5N)n is a fibrous 2-acetamido-2-deoxy-β-D-glucose where the repeated monomers are connected through β-1,4-glycosidic bond linkage. The numbering of the carbon atom was shown at the figure 8 and 9. Basically, the structure of chitin is similar to cellulose except for the replacement hydroxyl group at the carbon 2 with acetamido group (Kumar, 2000). Thus, in understanding more about the structure is same with cellulose.

Briefly, the chitin’s monomer ring is 4C1 chair conformation. There are two hydroxyl group at carbon number 3 and 5 (hydroxymethyl group). The acetamido group attached at the carbon number 2. Both of the hydroxyl group and acetamido group will form a hydrogen bond in same or neighbouring chain. Moreover, the present of acetamido group will increase the hydrogen bonding compare to the cellulose, thus give more strength.

Generally scientist has found three type of the polymorphic form of chitin that are α-, β-, and γ-chitin. In α-chitin, the arrangements of the chains are anti-parallel and result of the formation of orthorhombic from. In β-chitin, the monomers are arranged in parallel to each other, thus having monoclinic crystal symmetry. For the γ-chitin, the chain arrangement is the mixture of both α-, and β-chitin with two parallel chains in one direction and one in opposite direction (Fabritius et al. 2011).

4.0 Functions of Cellulose

Naturally, cellulose present in the plant, bacteria, fungi, and algae. In plant, it becomes the major component of the cell wall. Cellulose that present at those organisms are in the form of crystalline fibrils, called microfibrils with ~2 to 50 nm across and can reach until several μm3 depending on the organism that synthesize the cellulose. The crystallize microfibrils may exist in the form of cellulose Iα and Iβ and will organized further to form a layers, cell walls, and fibre (Wartz et al. 2010).

Artificially, there are several function of cellulose that has been reported. Cellulose can be used as renewable energy and bioproducts. Cellulose at the non-food plants like trees is a source of organic matter known as cellulosic biomass. The cellulosic biomass can be converted to bioenergy such as bioethanol and biodiesel. Beside that it also can form bioproducts such as biopolymer, biolubricant, biosurfactants, and biosolvent (Wartz et al. 2010).

Cellulose also take part in the pharmaceutical area. It has been used in the development of drug delivery system. Drug delivery system is the system in controlling the releasing of drug whether fast or slow after been introduced to the organism (Kamel et al. 2008).

In industrial field, it has been used as biodegradable packaging material due to its fascinating structure, biodegradable, biocompatible, and derivable properties (Yang et al. 2011). O’Connell et al. (2008) reported the use of cellulose as an adsorbant in removing heavy metal from the waste water.

5.0 Functions of Chitin


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Figure 10: Exoskeleton in American lobster Homarus americanus

Naturally, chitin is the basic component of outer lining for various type of organism such as the cell walls septa several group of fungi, bacteria, Antropoda, Annelida, Mollusca, and insect (Lenardon et al. 2010). By taking antropoda as an example, chitin is one of the main compounds in the exoskeleton of arthropoda that is the external skeleton that support and protect an animal’s body. Specifically, the exoskeleton is the hierarchical structure of cuticle which the basic material in the formation of the cuticle is the chitin that combines with various type of protein.

Figure 10 show the structure of the exoskeleton of American lobster Homarus americanus that was studied by Romano et al. (2007) and Sachs et al. (2006). The hierarchical structure can be observed to start with the chitin monomers (I) that polymerized to form a chitin structure (III). Then several chitin molecules are wrapped with protein (IV) and was organized further to form a layer of cuticle like the plywood structure (Fabritius et al. 2011).

Artificially, chitin has been used in variety of field same like cellulose. In the field of instrumentation, it has been reported that chitin was used in the preparation of affinity chromatography column (Rinaudo, 2006). Beside that, chitin also has been used in preparing various type of membrane for separation purpose (Uragami and Takura, 2006).

In pharmaceutical and medicinal, chitin was used in production of biomedical material. The properties of chitin such as low toxicity and biodegradable make it suitable to be used in medical treatment. Chitin was used in wound dressing material, drug delivery tools such as hydrogel (Kumar, 2000), and tissue regenerate process such as for regenerate hard tissue, bone, cartilage, and teeth (Uragami and Takura, 2006).

In agriculture, chitin plays a role in cell-stimulating materials. Soybeans that was coated with chitin derivatives compound show increase in the production such as in the rate of germination, pod number, plant dry weight, and crop yield (Kumar, 2000). Beside that, chitin can be blended with poly-ε-caprolactone to produce fibre and film which is suitable for use in agricultural defence film (Uragami and Takura, 2006).

6.0 Conclusion

Cellulose and chitin is two of the most abundant organic material in the earth. Because of their characteristics as a result of the structure, well research has been done to manipulate the compound and produce different type of valuable product. The functional group such as the hydroxyl group (cellulose and chitin) and acetamido group (chitin) make the compound be able to derives a derivatives compound that will further can create a new product.

7.0 References

1. Abe, K. and Yano, H. (2011). Formation of Hydrogels from Cellulose Nanofibers. Carbohydrate Polymers. 85(4): 733-737.

2. Fabritius, H., C. Sachs, C. Raabe, D., Nikolov, S., Friak, M., and Neugebauer, J. (2011). Chitin in the Exoskeletons of Arthropoda: From Ancient Design to Novel Materials Science. In  Gupta, N. S (Ed). Chitin. (pp. 35-60). Netherlands: Springer.

3. Kamel, S., Ali, N., Jahangir, K., Shah, S. H., and El-Gendy, A. A. (2008). Pharmaceutical Significance of Cellulose: A Review. Express Polymer Letters. 2(11): 758-778.

4. Kumar, R. M. N. V. (2000). A Review of Chitin and Chitosan Applications. Reactive & Functional Polymers. 46(1): 1-27.

5. Lenardon, D. M., Munro, C. A., Gow, and N. AR. (2010). Chitin Synthesis and Fungal Pathogenesis. Current Opinion in Microbiology. 13(4): 416-423.

6. Marguerite, R. (2006). Chitin and Chitosan: Properties and Applications. Progress in Polymer Science. 31(7): 603-632.

7. Mathews, K. C., Holde, V. K. E., and Ahern, K. G. (2000). Biochemistry. 3rd. ed. San Francisco: Addison Wesley Longman.

8. O’Connell, D. W., Birkinshaw, C., and O’Dwyer, T. F. (2008). Heavy Metal Adsorbants Prepared from the Modifications of Cellulose: A Review. Bioresource Technology. 99(15): 6709-6724.

9. Uragami, T. and Tokura, S. (2006). Material Science of Chitin and Chitosan. Japan: Kodansha.

10. Wertz, L. J., Bedue, O., and Mercier, P. J. (2010). Cellulose Science and Tecnology. Boca Raton: EPFL Press.

11. Yang, Q. L., Qin, X., and Zhang, L. (2011). Properties of Cellulose Films Prepared From NaOH/urea/zincate Aqueous Solution at Low Temperature. Cellulose. 18(3): 681-688.
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