Observations of Coherence in Bacterial Reaction Centers Using Two-Dimensional Electronic Spectroscopy

Abstract

Photosynthesis is the process by which plants and photosynthetic bacteria convert absorbed sunlight into usable chemical energy. In the first steps of photosynthesis, light energy absorbed by molecules embedded in photosynthetic proteins is rapidly transferred to a low-energy state in a type of protein called the reaction center. In the reaction center, this energy then converted into a charge separation which is followed by rapid and efficient movement of an electron out of the protein. Recent observations of coherent oscillatory signals (coherences) in photosynthetic proteins have been suggested to be responsible for the rapidity and efficiency of the energy and charge transfer processes in these systems. The reaction center of photosynthetic purple bacteria, the Bacterial Reaction Center (BRC), has long served as a model protein for understanding charge transfer processes due in part to the relatively well separated electronic peaks in its absorption spectrum and the availability of many mutants. Coherent oscillations previously observed in the BRC have been attributed to multiple conflicting origins. In this work we characterize the coherences present in the BRC using broadband Two-Dimensional Electronic Spectroscopy (2DES) with a nonlinear light source capable of generating pulses spanning the visible-NIR portion of the BRC spectrum. These 2DES experiments are some of the first to be performed on BRCs which undergo charge separation. Through comparison of the coherences in the BRC with a monomer of one of its main constituent pigments, Bacteriochlorophyll a (BChla), we assign multiple coherence origins, including those due to excited vibrational modes and those due to vibronic coupling between molecules inside of the BRC. The results presented in this thesis serve as the first direct comparison of monomeric BChla and BRC coherences. The coherence analysis detailed in this thesis presents several novel results. We significantly observe many prominent coherence modes in monomeric BChla; previous studies of coherence in this system have yielded conflicting reports of few or no coherences. We assign the observed BChla coherences to both excited and ground electronic state vibrational origins. We similarly observe a large number of coherence modes in two BRC mutants. These signals show strong signatures of vibrational coherence, similarly to in BChla, and additionally show signatures which are not explained by either purely vibrational or purely electronic origins. These signatures can be described by a mixed vibrational-electronic, or vibronic, model which has recently been used to describe coherences in a variety of photosynthetic proteins, including the BRC. We assign several of the signatures to the upper excitonic state of the strongly-coupled special pair, which has historically been difficult to resolve directly due to its low oscillator strength and proximity to other broad, strong transitions. The upper excitonic state is better resolved in this work due to vibronic coupling of the special pair states to neighboring monomeric BChla molecules. This vibronic coupling implies a stronger degree of coupling between these molecules than previously thought, providing a new perspective of the BRC as a system in which excitonic states are strongly delocalized over the special pair and monomeric BChla pigments. Accurate modeling of these signatures additionally requires inclusion of special pair charge transfer states. The results presented in this thesis should inform future efforts to model both Bacteriochlorophyll a and BRC electronic structures and the charge separation process in the BRC.