Date of Completion

7-31-2015

Embargo Period

7-31-2015

Keywords

carotenoids, photosynthesis, spectroscopy, light-harvesting complexes

Major Advisor

Harry A. Frank

Associate Advisor

Robert R. Birge

Associate Advisor

James F. Rusling

Field of Study

Chemistry

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

This thesis examines the factors affecting the photophysics and energy transfer properties of carotenoids. Steady-state and ultrafast time-resolved spectroscopic experiments were carried out on several carotenoids in various solvents and light-harvesting pigment-protein complexes. The main goal is to probe the excited state properties and kinetics of these molecules and relate the findings to their roles in light-harvesting and photoprotection of the photosynthetic apparatus. The spectroscopic studies on a short conjugated peridinin analogue extends previous studies on synthetic peridinin analogues having different numbers of conjugated carbon-carbon double bonds. The results provide insight into the nature of the intramolecular charge transfer state (ICT) in carbonyl-containing carotenoids. The spectroscopic properties of several light-harvesting pigment-protein complexes isolated from various photosynthetic organisms are also investigated. Some species of purple photosynthetic bacteria produce spectral variants of the well-known B800-850 light harvesting II (LH2) complex depending on the conditions under which they are grown. Rhodoblastus (Rbl.) acidophilus strain 7050 produces the B800-820 LH2 spectral variant when grown under low-light conditions. In addition, the carotenoid rhodopinal glucoside is formed in large amounts in this complex whereas rhodopin glucoside is the primary carotenoid in the B800-850 LH2 complex grown under high-light growth conditions. The conversion of rhodopin glucoside to rhodopinal glucoside increases the efficiency of carotenoid-to-bacteriochlorophyll (BChl) energy transfer to ~100%, as evidenced by results obtained from steady-state absorption, fluorescence, and ultrafast transient absorption spectroscopic measurements on rhodopin and rhodopinal glucoside in different solvents and in the LH2 complexes. Allochromatium (Alc.) vinosum is another example of a photosynthetic bacterium that produces various LH2 spectral forms denoted B800-850, B800-840 and B800-820 when grown under different conditions of temperature, illumination, and reduced sulfur nutrient. The analysis of the pigment composition reveals that the LH2 complexes from Alc. vinosum contain five carotenoids: lycopene, rhodopin, anhydrorhodovibrin, rhodovibrin and spirilloxanthin. Reconstruction of the absorption and fluorescence excitation spectra demonstrates that there exists significant spectral heterogeneity compared to LH2 complexes obtained from other species of purple bacteria. The combined results from these investigations provide insights into the mechanisms by which photosynthetic organisms adapt and survive under varying environmental conditions. The effect of the protein structure on the spectroscopic properties of carotenoids and (bacterio)chlorophylls (BChls) are also presented in this thesis. In higher plants, aggregation of the light-harvesting complex II (LHCII) has been postulated to be one of the factors affecting the rate and efficiency of the process of dissipation of chlorophyll (Chl) excess excitation energy known as nonphotochemical quenching. Spectroscopic measurements were performed on monomeric, trimeric and aggregated monomers and trimers of LHCII, the results of which reveal the differences in the excited state deactivation processes of the Chls and carotenoids bound in these complexes. The last chapter looks at the influence of protein structure on the spectroscopic properties of the protein-bound peridinin and Chl a molecules in three native and recombinant peridinin-chlorophyll a-protein (PCP) complexes from photosynthetic dinoflagellates. Analysis of the absorption and fluorescence excitation spectra reveal that the individual peridinins in the PCP complexes have distinct spectra depending on their location in the pigment-protein complex and that all of the carotenoids possess the same high (~100%) energy transfer efficiency to Chl.

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