Modification of Chitosan by Thiourea Dioxide

Abstract

Thiourea dioxide (TDO, aminoiminomethanesulfinic acid, formamidinesulfinic acid) was used for the chemical modification of chitosan. The interaction of TDO with chitosan in the presence of alkali results in the guanidinylated chitosan, the substitution degree is 0.25-0.27 and does not depend largely on molar ratio of thiourea dioxide to chitosan. The structure of modified chitosan has been proved using UV and IR spectroscopy as well as elemental analysis. It is shown that modification of chitosan proceeds under mild conditions. Contrary to chitosan, its guanidinylated derivative has biocidal properties against Gram-positive and Gram-negative bacteria in the aqueous solutions close to neutral (pH 6.2). It can be explained by the partial substitution of amino groups by guanidine groups existing predominantly in the protonated form in the neutral aqueous solutions. The system thiourea dioxide-hydrogen peroxide was used for the oxidative modification of chitosan. It is shown that thiourea dioxide and hydrogen peroxide separately do not oxidize chitosan but in the presence of their mixture the formation of carboxylic groups in chitosan has been observed. The quantity of carboxylic groups in the modified and native chitosan has been determined by the titration with sodium hydroxide. The presence of carboxylic groups has been proved also using IR spectroscopy. It is shown that the quantity of carboxylic groups increases with the increase of the ratio [TDO]/[chitosan].