Indirubine, formation from indigo

In the present case, this was accomplished with isatin-5-sulfonate instead of isatin, using the urea-Unked cyclodextrin dimer to alter the ratio of formation of indigo, through oxidative dimerisation of indoxyl, to indirubin-5 -sulfonate, from indoxyl and the isatin-sulfonate (Figure 3.5). The isatinsulfonate exists in solution in equilibrium with its hydrate, which deprotonates with a pA a of 9.55 to give the corresponding anion... 

Oxidation of indoles by hydrogen peroxide under weakly acidic conditions (72HC(25-l)l) yields 2,2-di(3-indolyl)indolin-3-one (160), indigo (161) and, to a lesser extent, indirubin. All of the products are derived from the initial formation of indolin-3-one by a mechanism... 

A problem connected with the microbial production of indigo is the formation of a small amount of isatin, which could be partly suppressed by adjusting the 02 concentration [108]. Isatin reacts with indoxyl to give indirubin, which gives the finished denim an undesirable red cast. The introduction of an isatin-de-grading enzyme (isatin hydrolase from P. putida) satisfactorily reduced the level of indirubin [107]. 

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