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Abstract

Photosynthetic pigment-protein complexes can capture and transport solar energy with high efficiency and possess photoprotective mechanisms to prevent excitations from damaging the cell. Over the last decade, mounting evidence has shown mixed electronic-vibrational, or vibronic, states play an important role in photosynthetic energy transfer dynamics. The principles by which photosynthetic complexes engineer the excitonic Hamiltonian can inform how to design energy transport capabilities in modern light harvesting devices. In this thesis, I study how evolution has leveraged vibronic coupling to tune the energy transfer capabilities of the Fenna-Matthews-Olson (FMO) protein complex from green sulfur bacteria. It has been shown that FMO can quench excess excitations in oxidizing redox environments to prevent generation of reactive oxygen species. To understand the influence of the redox environment on the sub-picosecond (<10-12 s) energy transfer dynamics, two-dimensional electronic spectroscopy (2DES) is used to study the excited state evolution of wild-type and cysteine deficient FMO complexes in oxidizing and reducing conditions. I develop an analysis method that leverages dynamical symmetries between diagonal and below diagonal cross peaks to deterministically fit to time constants for energy transfer. Two-dimensional spectra are simulated to confirm that the method is accurate for chlorophyll-based excitonic systems. The energy transfer rates between FMO samples show that that the cysteines steer excitations through different pathways in the complex depending on whether the redox conditions are oxidizing or reducing. In oxidizing conditions, excitations are funneled through a pathway to increase the probability of being quenched. Redfield theory modeling shows that the pigment site energies are tuned in oxidizing conditions such that an exciton energy gap falls out of resonant coupling with a pigment vibration. In other words, vibronic coupling is switched on in reducing conditions and off in oxidizing conditions. I further show that many long-lived excited state coherences are only present in reducing conditions when vibronic coupling is present. The coherences correlate with downhill energy transfer, as their magnitude is strongest at below diagonal cross peaks. Our spectral analysis reveals that they are mostly vibrational in character and evolve via coherence transfer. The vibrational coherences are maintained through the energy transfer process, which indicates that the vibronically-enhanced energy transfer in reducing conditions is a coherent process. These results are the first to show that a biological system has evolved the ability to modulate an exclusively quantum mechanical effect, vibronic coupling, in response to damaging environmental redox conditions.

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