Elsevier

Carbohydrate Research

Volume 324, Issue 4, 10 March 2000, Pages 268-274
Carbohydrate Research

A simple preparation of half N-acetylated chitosan highly soluble in water and aqueous organic solvents

https://doi.org/10.1016/S0008-6215(99)00263-3Get rights and content

Abstract

A simple and improved method of preparing highly soluble chitosan (half N-acetylated chitosan) was developed using a series of chitosan samples of low molecular weights, and the solubility of the half N-acetylated chitosan in water and organic solvents was investigated in detail. To reduce the molecular weight, chitosan was treated with NaBO3 under the condition that chitosan was homogeneously dissolved in aqueous acetic acid. Weight-average molecular weights of the obtained chitosan samples were determined using a size-exclusion chromatography system equipped with a low-angle laser light-scattering photometer. Each chitosan sample was then N-acetylated with acetic anhydride under the condition that chitosan was homogeneously dissolved in aqueous acetic acid again. The water solubility of the half N-acetylated chitosan thus prepared increased with decreasing molecular weight. From 1H NMR spectroscopy, it was suggested that the sequence of N-acetylglucosamine and glucosamine residues was random. The solubility of the half N-acetylated chitosan of low molecular weight was rather high even in aqueous dimethylacetamide and dimethylsulfoxide.

Section snippets

1. Introduction

Chitin, poly-β-(1→4)-N-acetyl-d-glucosamine, is the second most abundant natural polysaccharide and exists largely in the shells of crustacea and insects. Chitosan, poly-β-(1→4)-d-glucosamine, can be readily obtained from chitin by deacetylation with alkali. The use of chitin and chitosan in various functional materials, including biomedical materials, have recently been developed [1], [2]. We have also reported on the membrane applications of chitosan and its derivatives [3], [4], [5].

Materials

Commercially available chitosan from Tokyo Kasei Kogyo, Tokyo, Japan, was purified thrice by reprecipitation using aq AcOH (5%) and aq KOH in advance. It was finally washed with deionized water and acetone, and then dried in vacuo. Standard pullulans for size-exclusion chromatography (SEC) were purchased from Showa Denko, Tokyo, Japan. Acetic anhydride, acetone, N,N-dimethylacetamide (DMA), and Me2SO from Wako Pure Chemical Industries, Osaka, Japan, were distilled just before use. All other

3. Results and discussion

Although various methods are available for depolymerization of chitosan, we employed an oxidative degradation with NaBO3 to save time. Also aqueous acetic acid was used as the solvent to achieve the reaction under homogeneous conditions. Consequently, the main chain of chitosan seems to be randomly cut up. Table 1 lists the degradation conditions along with the weight-average molecular weight and the number-average degree of polymerization for the chitosan samples obtained. The weight-average

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