Biosynthesis of violacein: oxygenation at the 2-position of the indole ring and structures of proviolacein, prodeoxyviolacein and pseudoviolacein, the plausible biosynthetic intermediates of violacein and deoxyviolacein

During a search for biosynthetic intermediates of violacein pigments 1 and 2, employing enzyme inhibitors and blocked mutants, three novel tryptophan metabolites 3, 4 and 6 were found. Compounds 3 and 4, named prodeoxyviolacein and proviolacein, respectively, were red in neutral or basic medium, and blue in acidic solution. However, the red–orange colour of compound 6, named pseudoviolacein, did not change with acidity. The structures were determined by chemical modifications and by the analysis of HRMS (EI) and 2D NMR spectra resulting from COSY (1H–1H and 1H–13C), HMBC and NOESY experiments. The structures of 3, 4 and 6 were established as 3,5-di(indol-3-yl)pyrrol-2(2H)-one, 5-(5-hydroxyindol-3-yl)-3-(indol-3-yl)pyrrol-2(2H)-one and 5-(5-hydroxy-2-oxoindol-3-ylidene)-3-(indol-3-yl)pyrrol-2(1H)-one, respectively. The structure of 6 is similar to that of violacein 2, differing only in that the other indolyl group is oxygenated. Compounds 3 and 4 do not have an oxoindole unit. The isolation of 3, 4 and 6 together with 5(named as pseudodeoxyviolacein, previously published) lead to the proposition that the oxygenation at the 2-position of indole ring is a final step in the biosynthesis of violacein analogues.