Protozoan Pigment Puzzle Solved

It took some time, but chemists have finally deciphered the structure of the pigment that gives the protozoan Stentor amethystinus its unusual red-violet hue (J. Nat. Prod. 2014, DOI: 10.1021/np5001363). S. amethystinus is one of many brightly colored protozoans that live in lakes, ponds, and marine environments. Although the structures of these protozoan pigments have largely been elucidated over the years, the molecule responsible for S. amethystinus’s distinctive color has been a mystery. In August 1995, researchers led by Gerhard Höfle of Germany’s Helmholtz Centre for Infection Research made their first collection of blooms of S. amethystinus from a lake north of Brandenburg. From this material they were able to isolate just 755 mg of a dark red crystalline powder they dubbed amethystin. The researchers had to pause the project repeatedly for years because the organism did not appear regularly. Attempts to prepare crystals of sufficient quality for X-ray analysis failed, leaving the researchers to rely on spectroscopic methods and derivatization to determine the structure. In solution, they discovered, the compound exists as an orthoester (shown). But in the crystalline state, it can exist as a hy­droxy­lac­tone as well.