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Abstract

Photosynthetic organisms have adapted to survive in their diverse native habitats, whether by maximizing light-harvesting efficiency in low-light environments or by quenching excess excitations to prevent the formation of damaging reactive oxygen species. Photosynthetic species follow a number of principles in the design of pigment-protein light-harvesters, including the dense packing of chromophores, repeating the same pigment many times, and using interactions with other pigments and the environment to tune the absorbance spectrum. We seek to understand the ultrafast dynamics of light harvesting in these organisms that take place on the femtosecond timescale. In this thesis, I track energy moving through both energetically distinct and degenerate states by integrating ultrafast two-dimensional electronic spectroscopy with anisotropy and power-dependence experiments. I present results on the ultrafast energy transfer processes in the anoxygenic photosynthetic purple bacterium Rhodobacter sphaeroides and the oxygenic cyanobacterium Synechocystis sp PCC 6803 in their native membrane environments. Both of these organisms use light-harvesting antenna complexes to collect light energy and funnel that energy to an energetic trap known as the reaction center which produces a trans-membrane electron gradient that drives photosynthetic reactions. In Rhodobacter sphaeroides, we collected the first two-dimensional electronic anisotropy spectra, revealing time-dependent dynamics of the transition dipole orientations in membrane-bound light harvesting complex 2 (LH2). This pioneering spectroscopy reveals orientational preferences for sequential absorption processes, ultrafast intra-band equilibrations, and differentiates energy transfer processes from spectrally overlapped states that are otherwise unresolvable. We investigated light-harvesting pathways in LH2 from organisms grown under varied light and oxygen conditions to determine whether an energy quenching pathway was introduced. We present a revised picture of the B850 dynamics in LH2, showing power dependent dynamics that were previously thought to be inherent to the B850 ring. The first two-dimensional spectra of intact cyanobacterial thylakoid membranes are also presented here. We seek to disentangle the spectral contributions of the fused antenna/reaction center photosystems in Synechocystis sp PCC 6803 and map out inter-complex energy transfer. Mutations deleting specific portions of the photosynthetic machinery enable us to attribute spectral features and dynamics to specific complexes that would otherwise be indistinct. Future planned experiments include the investigation of cyanobacterial photoprotection and the effect of iron stress on energy transfer pathways.

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