Graduate Thesis Or Dissertation
Two-Dimensional Electronic Spectroscopy of the Baseplate in Green Photosynthetic Bacteria Public Deposited
In green photosynthetic bacteria, sunlight is absorbed by the chlorosome, andthen excitation energy is funneled through the baseplate and membrane complexes to the reaction center on timescales of tens to hundreds of picoseconds. The structure of the baseplate has been previously proposed to contain hundreds of pigment dimers held together within protein rods. In this work, the baseplate complexes from two species of bacteria, Chloroflexus aurantiacus and a mutant of Chlorobaculum tepidum, were studied with ultrafast two-dimensional electronic spectroscopy at 78 K.
By analyzing the lineshapes and changes in the 2D spectra as a function of population time delay, it was determined that downhill energy relaxation occurred on many timescales, accompanied by loss of excitations in both species. The isotropic signal decay, which measures loss of excited state population, is consistent with a model that incorporates unimolecular decay and one-dimensional diffusion of excitations along a baseplate rod that causes excitons to annihilate when they meet. The fastest timescale for anisotropy decay is 3 to 5 ps, which is not consistent with a model in which the entire baseplate consists of strongly coupled dimers. The anisotropy, the 2D spectra, and the isotropic signal decay are all consistent with a model in which the average excitation energy transfer hopping time is less than 1 ps and the average change in angle between transition dipoles is small for each hop.
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