Comparison of N-Hydroxy Compounds as Mediators in Laccase-Catalysed Decolorization of Indigo Carmine

A comparative study of N-hydroxy compounds - violuric acid, 1-hydroxybenzotriazole and N-hydroxyphthalimide as the redox mediators during laccase-catalyzed Indigo Carmine decolorization has been carried out. It is shown that the enzymatic oxidation of N-hydroxy-mediators to radicals >N-O-center dot, which act as a real oxidizing agent for indigo carmine, is a rate-limiting stage. The effectiveness of N-hydroxy compounds as the mediators in the Indigo Carmine decolorization in the presence of laccase Trametes versicolor increases in the following order: N-hydroxyphthalimide < 1-hydroxybenzotriazole < violuric acid. The main reason for the low activity of N-hydroxyphthalimide is the low rate of generation of phthalimide-N-oxyl radicals in the process of oxidation by laccase of N-hydroxyphthalimide, which is confirmed by inhibition of the studied process by ascorbic acid. The initiation rate corresponds to the total process rate and is equal to 4.20x10(-8), 2.82x10(-8), 2.90x10(-9) M s(-1) with the participation of violuric acid, 1-hydroxybenzotriazole and N-hydroxyphthalimide, respectively.