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 • , 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 4.20×10 −8 , 2.82×10 −8 , 2.90×10 −9 M s −1 with the participation of violuric acid, 1‐hydroxybenzotriazole and N ‐hydroxyphthalimide, respectively.