Spectral and kinetic properties of carotenoid triplet states in solution and in the peridinin-chloropyhll a protein complex. Synthesis of carotenoids for pigment identification and photochemical studies

Date of Completion

January 2011

Keywords

Chemistry, Physical

Degree

Ph.D.

Abstract

The research presented in this thesis is focused on how carotenoids protect the photosynthetic apparatus by quenching chlorophyll (Chl) triplet states before they can sensitize the formation of the highly reactive ¹Δg* excited singlet state of oxygen. Nanosecond time-resolved flash-transient optical spectroscopy was used to evaluate the effect of structural modifications and environment on the triplet state photophysics of peridinin in solution and in the peridinin-chlorophyll a-protein (PCP) from the dinoflagellate Amphidinium carterae. A series of peridinin analogues with increasing π-electron chain length and with different functionalities were studied in various solvents using Chl a as a triplet state donor. Measurements were also performed on native and reconstituted PCP complexes containing peridinin and various Chls. The data indicate that Chl triplet state trapping and hence, photoprotection of the PCP complexes, is accomplished via a delocalized triplet state involving both the bound peridinins and Chls. In seeking to evaluate the role of carotenoids in protecting the photosynthetic apparatus from the deleterious effects of singlet oxygen, it is of interest to know the yield and concentration of Chl and carotenoid triplet states formed upon photoexcitation. This requires a knowledge of the triplet state extinction coefficients, very few of which have been published for carotenoids. Thus, one chapter in this thesis focuses on a method for measuring the triplet state (T1 → Tn ) extinction coefficients of carotenoids, and the method is applied to all the major xanthophylls in green plants, zeaxanthin, neoxanthin, lutein and violaxanthin. The final part of this thesis describes full and partial syntheses of several carotenoids: β-12'-apocarotene,47 3,3'-dimethoxy-β,β-carotene-4,4'-dione (3,3'-dimethoxy-astaxanthin), 3,3'-dimethoxy-2,3,2',3'-tetradehydro-β,β-carotene-4,4'-dione (3,3'-dimethoxy-astacene) and 3,3'-dimethoxy-β,β-carotene (3,3'dimethoxy-zeaxanthin). β-12'-apocarotene which were needed to evaluate the photophysics of related retinal derivatives and to confirm the identification of pigments isolated from the feathers of an obligate frugivore avian species, Xipholena punecia. ^

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