Effect of hydrogen peroxide treatment on the molecular weight and structure of chitosan

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Abstract

The degradation of chitosan by hydrogen peroxide was studied. The degradation was monitored by gel permeation chromatography (GPC). The molecular weight distribution of partially degraded products from homogeneous reaction displayed a single-modal curve whereas that from heterogeneous reaction was bimodal. The molecular weight (Mw) decreased as the temperature, the time and/or hydrogen peroxide concentration increased. The dissolution of chitosan (pH 5.5) in minimum hydrochloride enhanced its degradation, but excessive hydrogen ion potentially inhibited the degradation. The formation of carboxyl group and deamination in the products indicated that the decrease of Mw was accompanied by structure changes. The degraded chitosans in Mw range from 51×103 to 1.2×103 were characterized by elemental analysis, X-ray diffraction, thermogravimetric analysis (TGA) and differential thermal analysis (DTA), Fourier transform infrared (FT–IR), carbon-13 nuclear magnetic resonance spectroscopy (13C-NMR). There was no significant chemical change in the backbone of chitosan with Mw of 51×103. But degraded chitosan with Mw of 3.5×103 had 1.71 mmol/g carboxyl groups and lost about 15% of amino groups. Further degradation led to more ring-opening oxidation of degraded products, the formation of carboxyl groups and faster deamination. The degraded chitosan with Mw of 1.2×103 had 2.86 mmol/g carboxyl groups and lost more than 40% of amino groups.

Introduction

Chitin, poly-β-(1→4)-linked-N-acetyl-d-glucosamine, is one of the most abundant, easily obtained, and renewable natural polymers, second only to cellulose. It is commonly found in the exoskeletons of crustaceans and in cell walls of fungi, insects and yeast [1]. Every year approximately 100 billion tons of chitin is produced on the earth. Chitosan, the deacetylated derivative of chitin, is one of the nontoxic and biodegradable carbohydrate polymers, and has received much attention as a functional biopolymer for diverse applications from pharmaceuticals to commodity chemicals [2], [3]. These functions undoubtedly depend upon not only their chemical structure but also the molecular size [4].

Hydrogen peroxide has long been used in the treatment of polysaccharides such as cellulose [5], starch [6] and hemicellulose [7] because it is easy to handle, easily available and environmentally friendly. Although the treatment of chitosan by H2O2 as a method for the preparation of chitosan with low molecular weight was reported [8], [9], [10], there are few papers on the relation between the molecular weight and the structure of the obtained products in detail. An understanding of chemical structure and molecular weight of resulting chitosan is essential for better application. This work is concerned with the degradation of chitosan using aqueous H2O2. Factors affecting the reaction are studied. The products with different molecular weights were comparatively investigated by GPC, FT–IR, 13C-NMR,TGA/DTA and elemental analysis as well as chemical analyses.

Section snippets

Materials

Chitosan (CS-0) from shrimp shell, whose degree of deacetylation was 90.5%, was purchased from Yuhuan Biochemical Co. (Zhejiang, China), and was used after passage through a 200 mesh sieve. All other chemicals were of reagent grade.

Characterizations

The number average molecular weight (Mn) and weight average molecular weights (Mw) were measured by gel permeation chromatography (GPC). The GPC system was a TSP P100 instrument. Two columns in series (TSK G5000-PW and TSK G3000-PW) were used. The eluent was 0.2 mol l

Reduction in Mw

Samples were taken at intervals during the degradation. Fig. 1 shows the GPC profiles of chitosans degraded with H2O2 in 0.9% HCl solution (Fig. 1a) and in distilled water (Fig. 1b) at 60 °C. The shift toward higher elution volumes as a consequence of the degradation could be observed for all samples. Obviously the extent of degradation increased by prolonging the duration. These profiles give information on the degradation, indicating that the degradation of the backbone occurred in a random

Conclusion

The treatments of chitosan with H2O2 led to a decrease in the degree of polymerization, even at ambient temperature. The macromolecular chitosan degraded most rapidly in the lowest acid concentration just for the formation of chitosan solution. Higher temperature and H2O2 concentration accelerated the degradation. Moreover, the treatments of chitosan with H2O2 also resulted in change in the chemical structure. The changes such as formation of carboxyl groups and deamination increased with the

Acknowledgements

The authors are grateful for the financial support of the National Natural Science Foundation of China and China Capital Investment, Ltd., in Shanghai to this research.

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