Spectroscopic properties of carotenoids in red and blue hues

Date of Completion

January 2006

Keywords

Chemistry, Physical|Biophysics, General

Degree

Ph.D.

Abstract

Photosynthesis is a process by which plants, algae, and photosynthetic bacteria convert light into chemical energy. Light-harvesting is accomplished by antenna pigment-protein complexes absorbing light and transferring the excitation energy to reaction center pigment-protein complexes where charge separation occurs. The two classes of pigments involved in light-harvesting are chlorophylls and carotenoids. Peridinin-chlorophyll a-protein (PCP) from the dinoflagellate, Amphidinium carterae is valuable system for studying light-harvesting because its structure has been resolved by X-ray crystallography. In this thesis, two variants of the PCP complex, main-form (MFPCP) and high-salt (HSPCP), have been investigated using steady-state and ultrafast transient optical spectroscopy at room and cryogenic temperatures. The systematic difference in the structures and pigment compositions of the MFPCP and HSPCP allows exploration of the factors that control energy transfer. Both the MFPCP and HSPCP complexes exhibit very high efficiency (> 95%) of peridinin-to-chlorophyll energy transfer. Spectroscopic studies of apo-PCP complexes reconstituted with peridinin and either chlorophyll a, chlorophyll b, chlorophyll d, 3-acetyl chlorophyll a and bacteriochlorophyll a were also carried out and found to exhibit very high energy transfer efficiencies. This further reveals the robust nature of the PCP complex; i.e. it can maintain a high energy transfer efficiency despite profound alterations in pigment composition. The factors controlling peridinin-to-chlorophyll energy transfer are the spectral overlap, distance between these pigments, and pigment-pigment and pigment-protein interactions. In this thesis, all of these factors are evaluated systematically. ^ In a complementary study, α-crustacyanin was investigated using steady-state and femtosecond transient absorption spectroscopy. α-crustacyanin is composed of eight β-crustacyanin and only binds the carotenoid, astaxanthin, which undergoes a large bathochromic spectral shift in the protein. The reason for the shift is described in this work. This system provides a means of understanding the intrinsic behavior of carotenoids in a protein environment in the absence of interactions with chlorophyls. Because the X-ray crystal structure of β-crustacyanin has been reported previously, this complex allows exploration of the relationship between structure, spectroscopic observables, and functions of protein-bound carotenoids.^

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