Excited States of Carotenoids and Their Roles in Light Harvesting Systems

KEŞAN, Gürkan

Carotenoids are an extensive group of natural pigments employed by a majority of organisms on earth. They are present in most organisms, including humans, but can be synthesized only by plants and microorganisms. They perform two major roles in photosynthesis, often in partnership with the more prevalent chlorophylls (Chls) / bacteriochlorophylls (BChls): light-harvesting and photoprotection. Carotenoids absorb radiation in a spectral region inaccessible to Chls and BChls and transfer the absorbed energy to (B)Chls which, in turn, funnel it into the photosynthetic reaction center (RC). It is determined that the structures and dynamics of excited states of carotenoids found in photosynthetic proteins provide an explanation of their roles as light-harvesting and photoprotective agents. The conjugation length and the type of carotenoids play a big part in understanding the energy transfer from carotenoids to Chls and BChls, because excited-state properties of carotenoids are affected by number of conjugated C=C bonds and their structures. An accurate description of these states is, therefore, the crucial first step in explaining carotenoid photochemistry and understanding the interactions between carotenoids and other molecules in photobiological processes. The research in this thesis, femtosecond ultrafast transient absorption spectroscopy was used to study the light-harvesting function of carotenoids both in solvent and protein environment. The findings were supported with computational methods. Based on spectroscopic indications, the light-harvesting function of carotenoids has a bearing on their structures, and specific light-harvesting strategies are explicitly dependent with the structure of the light-harvesting complexes.

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