The impact of sulfonation pattern on indigo degradation by Phanerochaete chrysosporium, ligninolytic enzymes

Indigo dyes can be successfully decolorized by both groups of Phanerochaete chrysosporium extracellular ligninolytic enzymes, i.e. by manganese (MnP) and lignin (LiP) peroxidases. The decolorization rate of indigo di-, tri-, and tetrasulfonate was measured using isolated isoenzymes LiPH2 and MnP2. Although the decolorization was successful by MnP and LiP on all three tested indigo sulfonates, the degree of indigo ring sulfonation had an important impact on the decolorization rate. In the case of MnP, the specific decolorization rate dropped by approximately one order of magnitude with each additional sulfonic acid in the dye structure. This diminution followed the increase of the dyes standard potential which accompanies a higher degree of sulfonation. Using the LiP isoenzyme the process was reversed; indigo tetrasulfonate was decolorized most rapidly, while the other two dyes were decolorized considerably more slowly. An analogy with the, HSAB Principle it is proposed to explain this unusual behavior observed in the decolorization of indigo sulfonates by LiP.