Synthesis of Dibenzo-24-crown-8 Conjugated Chitosan with Different Amide Bond Coupling Methods

Background: Chitosan is well-known as a metal chelating agent for removing metal ions in near-neutral conditions. Chemical modification of chitosan has attracted attention to improve its metalion adsorbent properties. Recently, dibenzo-24-crown-8 derivatives were synthesized and reported to chelate with metal ions such as cesium. However, to date, to the best of our knowledge, there is no report on the use of dibenzo-24-crown-8 to modify chitosan. In this paper, the modification of amino groups on chitosan by amide formation with dibenzo-24-crown-8 moieties in aqueous solution was investigated by three different methods: two dehydrative condensing reactions with dibenzo-24-crown-8 having a COOH group and the use of an activated ester derivative of dibenzo-24-crown-8. Methods: Three methods, i.e., the reactions of dibenzo-24-crown-8 having a COOH group with either EDC-1-hydroxybenzotriazole (HOBt) or EDC-N-hydroxysuccinimide (NHS) and the reaction using the succinimide-activated ester of dibenzo-24-crown-8, were compared on chitosan. The prepared chitosan- dibenzo-24-crown-8 conjugates were characterized by 1H NMR and IR spectroscopy. The degree of substitution (DS) by the dibenzo-24-crown-8 moieties at the amino groups on chitosan was calculated by 1H NMR analysis. Results: In both the EDC-HOBt and EDC-NHS methods, the maximum DS was only 4.1%. Conversely, a higher DS (15.2%) was afforded when the succinimide ester of dibenzo-24-crown-8 was reacted with chitosan under pH 6.0. The 1H NMR spectrum of the product showed signals from both chitosan and the crown ether. C=O bands attributable to the amide linkage between chitosan and the crown ether appeared at 1656 and 1552 cm−1 in the IR spectra. Conclusion: The use of the activated ester of dibenzo-24-crown-8 resulted in a higher DS at amino groups on chitosan when compared with the EDC-HOBt or EDC-NHS systems. This result implies that the use of activated ester derivatives is efficient for the modification of chitosan in aqueous solution.