Taurine bromamine: Reactivity of an endogenous and exogenous anti-inflammatory and antimicrobial amino acid derivative
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Taurine bromamine (Tau-NHBr) is produced by the reaction between hypobromous acid (HOBr) and the amino acid taurine. There are increasing number of applications of Tau-NHBr as an anti-inflammatory and microbicidal drug for topical usage. Here, we performed a comprehensive study of the chemical reactivity of Tau-NHBr with endogenous and non-endogenous compounds. Tau-NHBr reactivity was compared with HOBr, hypochlorous acid (HOCl) and taurine chloramine (Tau-NHCl). The second-order rate constants (k2) for the reactions between Tau-NHBr and tryptophan (7.7 x 102 M-1s-1), melatonin (7.3 × 103 M-1s-1), serotonin (2.9 × 103 M-1s-1), dansylglycine (9.5 × 101 M-1s-1), tetramethylbenzidine (6.4 × 102 M-1s-1) and H2O2 (3.9 x M-1s-1) were obtained. Tau-NHBr demonstrated the following selectivity regarding its reactivity with free amino acids: tryptophan > cysteine ~ methionine > tyrosine. The reactivity of Tau-NHBr was strongly affected by the pH of the medium (for instance with dansylglycine: pH 5.0,1.1 × 104 M-1s-1, pH 7.0, 9.5 × 10 M-1s-1 and pH 9.0, 1.7 × 10 M-1s-1), a property that is related to the formation of the dibromamine form at acidic pH (Tau-NBr2). The formation of singlet oxygen was observed in the reaction between Tau-NHBr and H2O2. Tau-NHBr was also able to react with linoleic acid, but with low efficiency compared with HOBr and HOCl. Compared with HOBr, Tau-NHBr was not able to react with nucleosides. In conclusion, the following reactivity sequence was established: HOBr > HOCl > Tau-NHBr > Tau-NHCl. These findings can be very helpful for researchers interested in biological applications of taurine haloamines.