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Multidimensional Ultrafast Spectroscopy of Photosynthetic Pigment-Protein Complexes


This dissertation presents the application of ultrafast spectroscopy to the investigation of pigment-protein complexes (PPCs) involved in energy transfer and energy dissipation in photosynthetic organisms. PPCs are the building blocks of all photosynthetic organisms, and within individual pigment-protein complexes, energy transfer dynamics occur over fast timescales and broad spectral regions. Chapter 1 gives an introduction to the capability of photosynthetic organisms to absorb light energy, funnel this energy to a location for charge separation, and perform charge separation with subsequent conversion into chemical energy. All of these functions need to be performed efficiently under different light conditions. This chapter also includes a discussion of the different light harvesting strategies that various photosynthetic organisms have evolved. Lastly, this chapter discusses the application of ultrafast spectroscopy as an excellent tool to study the dynamics of PPCs. The three experimental techniques used in this dissertation are introduced in this chapter.

Chapters 2 and 3 present the application of two ultrafast spectroscopic techniques, two dimensional electronic spectroscopy and transient absorption spectroscopy, to the investigation of the structure-function relationship of photosynthetic pigment-protein complexes. Chapter 2 is an investigation of the photophysics of the two forms of the Orange Carotenoid Protein, involved in excess energy dissipation in cyanobacteria. Ultrafast spectroscopy allows us to understand how a conformational change that modifies pigment-protein interactions is able to generate a different biological function. Chapter 3 presents degenerate and non-degenerate two dimensional electronic spectroscopy experiments to investigate the energy transfer dynamics and couplings in the bacterial reaction center, which is the location of charge separation.

Chapter 4 investigates a PPC involved in excess energy dissipation in green algae, LhcSR. Fluorescence lifetime spectroscopy is used to investigate the excited state properties of LhcSR, which plays a role as light harvester, pH sensor and quencher in algae. The results allow us to identify two conformations, a light harvesting one and a quenching one. Transient absorption spectroscopy is used to investigate the energy transfer pathways involved in quenching in the two different conformations. Finally, Chapter 5 presents conclusions and extensions to future work, in order to improve our understanding of the energy transfer dynamics in natural photosynthetic systems.

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