Visible-light photochemistry and phototoxicity of thiarubrines

Article abstract of DOI:10.1111/j.1751-1097.1999.tb07984.x

Thiarubrines, a group of intensely red, sulfur-containing pigments produced by asteraceous plants, are photolabile, yielding thiophenes and elemental sulfur upon exposure to UV or visible light. The mechanism of this light-induced conversion for thiarubrines A (1a), B (1b) and D (1c), isolated from the roots of Ambrosia chamissonis (Asteraceae), was investigated. Visible-light irradiation of thiarubrines resulted in the formation of novel 2,6-dithiabicyclo[3.1.0]hex-3-ene polyyne photointermediates (photosulfides) that rapidly undergo desulfurization to yield thiophenes. Six photosulfides, photosulfides 3a and 3a′ from thiarubrine A (1a), photosulfides 3b and 3b′ from thiarubrine B (1b) and photosulfides 3c and 3c′ from thiarubrine D (1c) were characterized. Thiarubrine photointermediates are short-lived and unstable, with the photosulfides formed from thiarubrine A having a half-life of 12.3 min at room temperature. While the immediate fate of the extruded sulfur is unknown, we identified cyclooctasulfur (S₈) in photolysis solutions of thiarubrine A using electron impact mass spectrometry. Visible-light irradiation of Candida albicans cell suspensions treated with thiarubrine A led to a 99% decrease in cell viability, suggesting that the photosulfides, or other molecules generated by the exposure of thiarubrines to light, have significant toxicity.