The influence of L-cysteine and N-acetyl-L-cysteine on the stability and reducing activity of thiourea dioxide (NH2)2CSO2 (TDO) in the reaction with azo dye Orange II has been studied. The addition of L-cysteine and N-acetyl-L-cysteine leads to the increase in reducing activity of TDO in weakly acidic, neutral and weakly alkaline solutions, but does not influence its reducing activity in strongly alkaline solutions. In alkaline solutions, the rate of reduction of Orange II is increasing, but the influence of additives of thiol acids is decreasing. The activating effect of N-ace-tyl-L-cysteine is weaker than the effect of L-cysteine, that can be explained by the differences of pKa of these acids. Indeed, the reducing activity of thiols depends on the concentration of thiolate-ions in solutions. Since pKa of cysteine (8.30) is significantly less than pKa of N-acetyl-L-cysteine (9.52), concentration of thiolate-ions in cysteine solutions is higher. Therefore, reducing activity of L-cysteine is higher. The influence of L-cysteine and N-acetyl-L-cysteine on the reaction of TDO with Orange II is different from the influence of aminoacid glycine: in neutral solutions glycine decreases the rate of reaction. The interaction of TDO with L-cysteine in weakly acidic, neutral and weakly alkaline solutions is accompanied by the oxidation of L-cysteine to cysteinesulfenic acid and reduction of TDO to thiourea monoxide (NH2)2CSO. Then cysteinesulfenic acid reacts with L-cysteine with formation of cystine. The reaction studied here is the first example of reduction of TDO by sulfur-containing compounds.
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].
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.