Using Nature’s polyenes as templates: Studies of synthetic xanthomonadin analogues and realising their potential as antioxidants

Two truncated analogues of the polyenyl photoprotective xanthomonadin pigments have been synthesised utilising an iterative Heck-Mizoroki/iododeboronation cross coupling approach and investigated as models of the natural products as photoprotective agents in bacteria. Use of this methodology enabled the construction of a key unsubstituted tetraenyl iodide building block, despite the instability of this type of compound. Both analogues proved to be sufficiently stable to allow isolation, spectroscopic analysis and biological studies into their photoprotective behaviour. Analysis of the spectroscopic properties of these analogues provided support for the limited structural characterisation previously reported for some of the xanthomonadins and also fitted their expected behaviour despite their shorter polyene chain length. This behaviour also extended to their ability to protect bacteria from photochemical damage: incorporation of one compound into E. coli provided photoprotective activity against singlet oxygen analogous to the natural photoprotective mechanisms employed by Xanthomonas bacteria. This answers some key questions about what minimal functionality is required to impart photoprotection and opens up an exciting avenue of research into new classes of photoprotective and antioxidants.