Visible-light photochemistry and phototoxicity of thiarubrines

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-in duced conversion for thiarubrines A (1a), B (1b) and D (1c), isolated from the roots of Ambrosia chamissonis (Asteraceae), was investigated. Visible-l ight irradiation of thiarubrines resulted in the formation of novel 2,6-dit hiabicyclo[3.1.0]hex-3-ene polyyne photointermediates (photosulfides) that rapidly undergo desulfurization to yield thiophenes. Six photosulfides, pho tosulfides 3a and 3a' from thiarubrine A (1a), photosulfides 36 and 3b' fro m thiarubrine B (1b) and photosulfides 3c and 3c' from thiarubrine D (1c) w ere characterized. Thiarubrine photointermediates are short-lived and unsta ble, 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 sul fur is unknown, we identified cyclooctasulfur (S-8) in photolysis solutions of thiarubrine A using electron impact mass spectrometry, Visible-light ir radiation of Candida albicans cell suspensions treated with thiarubrine A l ed 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.