Tyrian purple: from mollusc to purple
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.
|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.|
|Now the tyrindoleninone reacts with tyrindoxyl to give tyriverdin which is yellow, but always looks green because ...|
|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.|
|Some of the tyriverdin is oxidised to give yellow 6-bromoisatin which reacts with tyrindoxyl to give 6,6'-dibromoindirubin which is reddish-purple.|
|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.|
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