Complex Kinetics in the Reaction of Taurine with Aqueous Bromine and Acidic Bromate : A Possible Cytoprotective Role against Hypobromous Acid

The most abundant aminoacid in the human body, 2-aminoethanesulphonic acid (H2NCH2CH2SO3H), is surprisingly stable and reacts exceedingly slowly even with the most powerful oxidizing agents like acidic bromate. Oxidation occurs only on the nitrogen centre to give the corresponding N-derivatives. No activity is observed at the sulphonic acid group and no cleavage of the C-S bond is observed. The stoichiometry of the oxidation of 2-aminoethanesulphonic acid by bromate is complex, yielding a mixture of monobromo- and dibromotaurines, oximes as well as the corresponding dimeric azo-compounds. In the presence of added bromide, the stoichiometry of the reaction is : 2BrO3- + 3H2NCH2CH2SO3H + 6H+ + 4Br- -> 3Br2NCH2CH2SO3H + 6H2O. Monobromotaurine is formedas an intermediate product before formation of the dibromotaurine. Aqueous bromine reacts quantitatively with 2-aminomethanesulphonic acid according to the stoichiometry : H2NCH2CH2SO3H + 2Br2 -> Br2NCH2CH2SO3H + 2Br- + 2H+. This reaction is strongly inhibited by acid due to the deactivation of the amino group to electrophilic attack by protonation. The formation of N-bromotaurines is suggested as a possible mechanism by which taurine can moderate the oxidative toxicity of bromine and hypobromous acid in the slightly basic physiological environments. South African Journal of Chemistry Vol.55 2002: 136-14