Development of ultrafast laser systems with applications to carotenoids in photosynthetic energy transfer

Date of Completion

January 2007

Keywords

Chemistry, Biochemistry|Biophysics, General

Degree

Ph.D.

Abstract

In the first part of this thesis, the design of a femtosecond Ti:Sapphire laser oscillator and laser amplification system including stretcher, multipass amplifier, and compressor is described, with particular attention to the dispersion compensation of the laser amplification system. In addition, the design of an achromatic degenerate third order autocorrelator is presented as it represents an important tool for evaluating the quality of the laser pulses. In the second part, the ultrafast optical spectroscopic properties of several carotenoids were studied. Carotenoids are comprised of two subgroups, xanthophylls and carotenes, and play numerous important roles in nature. These studies seek to unravel the complex photophysics of the molecules and discover the mechanism by which they function as harvesters of solar energy and regulators of energy flow in photosynthesis. Three xanthophylls, violaxanthin, lutein, and zeaxanthin; five open-chain carotenoids, neurosporene, spheroidene, rhodopin glucoside, rhodovibrin, and spirilloxanthin; and four light-harvesting pigment-protein complexes, denoted LH2 complexes, prepared from four different strains of photosynthetic bacteria, Rhodobacter (Rb.) sphaeroides GIC containing neurosporene, Rb. sphaeroides 2.4.1 (anaerobically grown) containing spheroidene, Rb. sphaeroides 2.4.1 (aerobically grown) containing spheroidenone, and Rps. acidophila 10050 containing rhodopin glucoside were studied. Experiments employed steady-state absorption, fluorescence, fluorescence excitation and ultrafast transient absorption spectroscopy. High-performance liquid chromatography (HPLC) was carried out immediately prior to the spectroscopic experiments to obtain highly pure molecules of the xanthophylls and open-chain carotenoids. Column chromatography using DEAE Sephacel (Sigma-Aldrich) of protein extracts from the bacteria was carried out to purify the LH2 complexes. Ultrafast optical spectroscopic experiments on the xanthophlylls were carried out at cryogenic temperatures in ether/isopentane/ethanol (EPA) glasses which provided enhanced spectral resolution compared to room temperature studies previously done. Methods of global fitting analysis were developed to analyze the spectral and temporal datasets. The data were found to be consistent with a model partitioning energy flow among various excited states including S1 (11A g-), vibronically excited S1 (11A g-), S2 (11Bu +) and the nebulous, S*. The spectral properties of the all- trans open-chain carotenoids were investigated in acetone and CS2 solution at room temperature to explore the effect of solvent and in EPA glasses at 77K, using steady-state absorption and ultrafast transient absorption spectroscopy in the visible region. The data support a model for S* as an excited state having a twisted conformational structure, the yield of which was increased in molecules having elongated systems of π-electron conjugation. The experiments were backed up by theoretical quantum computations done by Professor Robert Birge as a collaborator. Steady-state absorption, fluorescence, fluorescence excitation, and ultrafast transient absorption spectroscopy in both the visible and near-infrared (NIR) regions were carried out on LH2 complexes containing carotenoids with systematically increasing numbers of π-electron conjugated double-double bonds. One goal of this part of the work is to explain the structural features of carotenoids that control the rate and efficiency of energy transfer from carotenoids to bacteriochlorophyll in photosynthetic pigment-protein complexes. ^

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