Spectroscopic and photochemical properties of geometric isomers of carotenoids

Date of Completion

January 2005


Chemistry, Biochemistry|Chemistry, Physical




Photosynthesis is the process by which plants convert light into chemical energy. It is one of the important biological processes that sustains life on earth. Carotenoids are molecules synthesized by photosynthetic organisms and perform several functions in photosynthesis. They protect against the irreversible photodestruction of the photosynthetic apparatus and function as light-harvesting pigments, energy flow regulators, redox cofactors, and stabilizers and facilitators of protein assembly. Different geometric isomers of carotenoids have been found to exist in nature but the relationship between the structure and biological functions of geometric isomers of carotenoids is not fully understood. In this thesis, steady-state and time-resolved transient absorption spectroscopic studies have been carried out on unlocked and locked- cis geometric isomers of carotenoids. The molecules investigated are all-trans-spheroidene, 15,15'-locked-cis-spheroidene, 13,14-locked-cis-spheroidene, 11,12-locked-cis-spheroidene, all-trans-β-carotene, and 15,15'-cis-β-carotene. These molecules represent a systematic series for exploring the relationship between isomer geometry, spectroscopy, photophysics and biological function. The locked-cis molecules particularly are important because they are incapable of undergoing cis-to-trans isomerization. The experimental studies are supported by theoretical calculations which predict the relative oscillator strengths of the spectroscopic transitions. Femtosecond and nanosecond time-resolved spectroscopic studies of the excited singlet and triplet states of geometric isomers revealed only small changes in spectra and dynamics of the various molecules. This study is significant because it reveals the reason for the natural selection of geometric isomers of carotenoids in different pigment protein complexes and addresses the question whether isomer geometry plays a role in controlling the spectroscopy, photophysics and biological functions of carotenoids. ^