Visible-light Photochemistry and Phototoxicity of Thiarubrines (original) (raw)

Abstract

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 (la), B (lb) and D (Ic), 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 (la), photosulfides 3b and 3b' from thiarubrine B (lb) and photosulfides 3c and 3c' from thiarubrine D (lc) were characterized. Thiarubrine photointermediates are short-lived and unstable, with the photosulfides formed from thiarubrine A having a halfl i e 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.

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