The photochemistry and photophysics of carotenoids in photosynthesis

This work focused on the structures and photochemical properties of carotenoids. A major objective was to understand the mechanism of quenching of chlorophyll (Chl) fluorescence by carotenoids. Four carotenoids, $\beta$-carotene, lycopene, violaxanthin and zeaxanthin, have been chosen because of their biological importance and because they differ systematically in their structure and $\pi$-electron conjugation. The study evaluated the dependence of the positions of the excited state energy levels and the structure of carotenoids on Chl fluorescence quenching. Steady state fluorescence quantum yields and time-resolved fluorescence decay profiles of Chl were measured for the carotenoids at different concentrations. A model for the quenching of Chl fluorescence was presented that explains both the steady state and dynamics data.^ Another objective was to understand the properties of spheroidene which lead to isomerization following thermal excitation. This was investigated using high performance liquid chromatography (HPLC) and electronic spectroscopy in combination with theoretical calculations. Nuclear magnetic resonance spectroscopy was used to elucidate the structures of the geometrical isomers of spheroidene.