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Photobiology, photochemistry and chemical ecology of thiarubrines from the asteraceae Page, Jonathan Edward.


This study investigated several aspects of the biology and chemistry of thiarubrines, a group of red, light-sensitive pigments found in the Asteraceae. Irradiation of thiarubrines with visible light resulted in the formation of 2,6-dithiabicyclo[3.1.0]hexene polyyne photointermediates ("photosulfides"), which were shown to be short-lived and unstable, yielding thiophenes and cyclooctasulfur upon standing at room temperature. The structures of six novel photosulfides, formed by photolysis of thiarubrines A, B and D, were elucidated using HPLC with UV-vis and mass spectral detection, and cold-temperature NMR spectroscopy. Thiarubrine A showed visible-light phototoxicity to Candida albicans suggesting that the photosulfides, or other molecules generated by the exposure of thiarubrines to light, have significant toxicity. Investigation of the photoprotection of light-sensitive thiarubrines in the leaves of Ambrosia chamissonis revealed that anthocyanin pigments function as visible light screens that prevent thiarubrine photoconversion in vivo. Two anthocyanins, cyanidin 3-0-(6-0- malonyl-β-D-glucopyranoside) and cyanidin 3-0-(β-D-glucopyranoside), were isolated from A. chamissonis. Conversion of thiarubrines to thiophenes was shown to occur in A. chamissonis roots, which lack anthocyanins, but was prevented in leaves and stems, which contain high amounts of these light-screening compounds. Furthermore, solutions of cyanidin 3-0-(β-D-glucopyranoside) effectively prevented photoconversion of thiarubrine A in vitro. The hypothesis that thiarubrine A is a biologically-active constituent of the leaves of Wedelia (formerly Aspilia) species swallowed whole by chimpanzees was tested. Leaf samples were analyzed using a quantitative HPLC assay and a bioassay for toxicity to Candida albicans. Thiarubrines A, B, D and E and their corresponding thiophenes were not present in the leaves of four Wedelia species including W. mossambicensis, W. rudis and W. pluriseta, which are swallowed by chimpanzees at Mahale Mountains National Park and Gombe National Park, Tanzania. Candidicidal activity, indicative of the presence of thiarubrines, was also not detected in Wedelia leaf samples. A novel 1,2-dithiin polyyne, thiarubrine M, was isolated from the roots of Ambrosia chamissonis and its structure determined by UV, TR, NMR and mass spectroscopy. Thiarubrine M, which contains a 1,2-dithiin group coupled to a terpenoid derived side-chain, is of mixed biosynthetic origin. Unlike other thiarubrines, thiarubrine M was found to lack significant antifungal activity.

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