Enhancement of antioxidant activity of chitosan by irradiation
Introduction
Chitosan, an amino polysaccharide, has received much attention as a functional biopolymer for many diverse applications in food, pharmaceutical and cosmetics (Kumar, 2000, Shahidi et al., 1999). In many of these applications, specific molecular weights of polysaccharides are required. Chitosan with an average Mw in the range of 5000–10,000 Da possesses strong bactericidal and superior biological activities (Kittur, Vishu Kumar, & Tharanathan, 2003). Chitosan of 20 kDa prevents progression of diabetes mellitus and exhibits higher affinity for lipopolysaccharides than 140 kDa chitosan (Kondo, Nakatani, Hayashi, & Ito, 2000). Chitooligomers have special antimicrobial activity (Begona and Ruth, 1997, Zheng and Zhu, 2003) and antitumour activity (Qin, Du, Xiao, Li, & Gao, 2002). So it is of increasing interest to degrade chitosan into low molecular weight fragments under appropriate conditions. It has been reported that chitosan can be degraded by acidic hydrolysis or enzymatic treatment. Chemical treatment is an easy, low cost process, but chemical waste and reproducibility are the main problems. Enzymatic hydrolysis is an effective way to achieve specific cleavage to chitosan oligomers. However, it requires multi-steps, particularly, enzyme preparation and purification of the product.
Radiation can provide a useful tool for degradation of different polymers. In the reaction, no other chemical reagents are introduced and there is not a need to control the temperature, environment or additives (Charlesby, 1981). Recently, radiation effects on carbohydrates such as chitosan, sodium alginate, carrageenan, cellulose, pectin have been investigated to enhance their bioactivities and to reduce the environmental pollution (Chmielewski, Haji-Saeid, & Ahmed, 2005). Matsuhashi and Kume (1997) reported that chitosan degraded by irradiation could increase its antimicrobial activity as a result of a change in molecular weight. Chitosan irradiated at 100 kGy under dry condition inhibited the growth of Escherichia coli completely. Czechowska-Biskup, Rokita, Ulanski, and Rosiak (2005) studied the fat-binding capacity of irradiated chitosan and irradiation has proved useful in improving fat-binding properties of chitosan as an active component of dietary food additives – one gram of irradiated chitosan may bind up to 20 g of fat. Tham et al. (2001) showed that irradiated chitosan is effective as a plant growth promoter and heavy metal eliminator in crop production of plants stressed with vanadium.
Recently, the antioxidant activity of chitosan and its derivatives has attracted the attention. Since they exert strong antioxidant activities and their effects are also similar to those of phenolic antioxidants (Park, Je, & Kim, 2004). In this study, chitosan has been modified by irradiation to enhance its antioxidant activity, and the relation between the antioxidant activity and molecular properties of irradiated chitosan are discussed.
Section snippets
Materials
Chitosan, as initial material from shrimp shells, was obtained from Yuhuan Biochemical Co. (Zhejiang, China). Linoleic acid, nitroblue tetrazolium were purchased from Sigma Chemical Co. (St. Louis, USA). All other chemicals and reagents used were of analytical grade.
Irradiation
Chitosan (2 g) was dissolved in 1% (v/v) acetic acid solution (100 mL), and then the solution was irradiated by Co-60 γ rays at doses of 2, 10, 20 kGy.
Characterization
Weight-average molecular weight (Mw), number-average molecular weight (Mn) and
Effect of irradiation dose on Mw and DD
In the GPC curves of chitosan and its degraded products (Fig. 1), the peak of the elution curve shifted toward higher elution volumes with increasing irradiation dose. Irradiation induced scissions of 1–4 glycosidic bonds of chitosan polysaccharide that caused a reduction in the molecular weight of the polymer. GPC measurements showed a rapid decrease of molecular weight at the beginning of the degradation process, indicating that the degradation of the backbone mainly occurred in a random
Conclusions
Chitosan has two hydroxyl groups and one amino group in each of its monosaccharide construction units. The hydroxyl groups in the polysaccharide units can react with free radicals by the typical H-abstraction reaction (Xue, Yu, Hirata, Terao, & Lin, 1998). On the other hand, according to free radical theory, the amino groups in chitosan can react with free radicals to form additional stable macroradicals. Therefore, the active hydroxyl and amino groups in the polymer chains are the origin of
Acknowledgement
This research was supported by the grant (SB0404) of Guangxi Key Laboratory of Subtropical Bioresource Conservation & Utilization.
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