Date of Award
Doctor of Philosophy (PhD)
The green sulfur bacteria (class Chlorobea) are obligately anaerobic photoautotrophic prokaryotes. Members of this bacterial class are found in anoxic, sulfur- or iron-rich environments such as hydrothermal vents and hot springs. These bacteria are champions of lowlight adaptation, with some species being able to thrive photosynthetically on less than eight photons hr-1 per pigment. We have studied the two photosynthetic antenna complexes from these organisms that are responsible for light capture: the chlorosome and the Fenna-Matthews-Olson (FMO) protein.
In regards to the chlorosome, we studied, using steady-state and time-resolved spectroscopic methods, the effect of incorporation of an expected, but as-yet-undiscovered-in-nature, light-harvesting pigment into the complex, Bacteriochlorophyll (BChl) f. We show that BChl f is a fully functional pigment in chlorosomes, although it transfers energy less efficiently to the next energy acceptors in the system. Additionally, we have pioneered methods to build self-assembling biohybrid chlorosome analogs that incorporate natural pigment and synthetic polymers that mimics the natural chlorosome structure. These biohybrid constructs look and act like natural chlorosomes, although their design offers flexibility of pigment choice not found in nature.
In regards to the FMO protein, we studied the effect of excitation intensity on the singlet and triplet excited states of the BChl a molecules in the protein using ultrafast spectroscopy. We show that the system undergoes intersystem crossing into the triplet state after ~25% of excitations, and the lifetime of the triplet state is between 10-100 μs. We also, using electrochemistry and mass spectrometry, identified the source of the FMO protein's modulation of efficiency with respect to redox condition. One or both of the redox-sensitive cysteine residues in the protein, situated near BChl a #'s 2 and 3 modulate their redox state between free thiol and thiyl radical form to quench BChl excitations, probably via an electron transfer/ultrafast recombination mechanism. These results together help us to understand the molecular mechanisms of photosynthetic and environmental robustness in these bacteria more completely.
Chair and Committee
Robert E Blankenship
Dewey Holten, Michael L Gross, Timothy A Wencewicz, Robert G Kranz
Orf, Gregory Scott, "Light-Harvesting, Redox Control, and Biomimicry of the Photosynthetic Antennas of Green Sulfur Bacteria" (2015). Arts & Sciences Electronic Theses and Dissertations. 447.