Tyrian purple: from mollusc to purple

The Tyrian purple colour from molluscs is mainly composed of 6,6'-dibromoindigo, with smaller amounts of 6,6'-dibromoindirubin and 6-bromoindigo.   These coloured compounds are not present in the mollusc, but are the result of some quite complicated chemistry.   This is how it happens ...
In many molluscs, like Murex brandaris, Dicathais orbita and Nucella lapillus, the chemical precursor in the hypobranchial gland is a colourless indoxyl sulfate substituted in the 2-position by a methylthio group.   It has a trivial name: tyrindoxyl sulfate.

When the gland is removed and the cells begin to die, a purpurase or sulfatase enzyme removes the sulfate group to give tyrindoxyl as shown on the right.

hydrolysis of tyrindoxyl sulfate
Tyrindoxyl is easily oxidised to give tyrindoleninone which is red.   Some of the tyrindoleninone reacts reversibly with methanethiol, also present, to give the yellow tyrindolinone. tyrindoxyl reacts
Now the tyrindoleninone reacts with tyrindoxyl to give tyriverdin which is yellow, but always looks green because ... tyriverdin forms
Tyriverdin decomposes in sunlight to give 6,6'-dibromoindigo and the highly odorous dimethyl disulfide.   It looks green because there is always a little dibromoindigo present after it has been exposed to the light.   In complete darkness, it would be yellow, but rather difficult to see. dibromoindigo at last
Some of the tyriverdin is oxidised to give yellow 6-bromoisatin which reacts with tyrindoxyl to give 6,6'-dibromoindirubin which is reddish-purple. dibromoindirubin
Finally, to account for the 6-bromoindigo: Some of the 6,6'-dibromoindigo is in the reduced (leuco) form.   This leuco-6,6'-dibromoindigo is easily debrominated by light and the resulting leuco-6-bromoindigo is oxidised by the air to 6-bromoindigo. photodebromination

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