Primary Electron Transfer Processes in Photosynthetic Bacterial Reaction Centers

Washington University, Saint Louis MO

Investigators

Abstract

The ultimate goal of this research is to achieve a unified molecular level understanding of the photochemical processes in the bacterial photosynthetic reaction center, which is a pigment-protein complex found in photosynthetic bacteria. Plants have similar photosynthetic machinery. All these reaction centers have in common two quasi-symmetric branches of potential electron carriers. In the bacterial reaction center, only one of them, the so-called A-branch, is utilized for storing the energy of (sun)light -accomplished by trans-membrane photo-induced electron transport on the A branch. In the native reaction center, the B-branch is inactive. The previous work in this project has shown that under certain unique conditions, the B-branch cofactors can be induced to support full trans-membrane electron transfer. The yield is high enough that detailed explorations of electron transfer along the entire normally inactive B-branch of reaction center cofactors are now possible. Such studies are the basis of this project. This research will test current models and exploit insights gained from previous studies of the active-branch (A-branch) processes and in doing so elucidate mechanistic underpinnings of electron transfer in the RC as a unified whole. From a more global perspective, this project is aimed at making the normally inactive B-branch cofactor chain fully accessible in a robust manner for a variety of next-generation studies. This work will open up new avenues for exploring electronic and/or conformational changes, proton movement and other similar processes that are thought to be functionally significant in association with the QA/QB two-electron gate function of the two quinones (Qs) in the reaction center. These goals will be pursued via static and time-resolved (femtoseconds to seconds) spectroscopic studies of RC mutants with multiple amino acid changes that rationally manipulate the properties of the cofactors and the rates and yields of the charge separation and recombination processes. This research is not only central to understanding the primary photochemical events in the bacterial RC, but also will provide guideposts for research being conducted on plant photosystems I and II. Broader Impact: Understanding the molecular-level mechanisms of the processes in the photosynthetic RC lays a foundation for synthetic systems for solar-energy harvesting/conversion, thereby addressing a national need for next generation renewable energy sources. Undergraduate and graduate students will participate in this research. The integration of research ideas into teaching and educational development will focus on two projects. (1) A web-based tutorial will be developed. It will be tailored for undergraduate or high school students, on the general topic "Why is grass green and blood red?" (2) Two undergraduate physical-chemistry laboratory experiments at Washington University will be extensively upgraded to exploit the spectroscopy of chlorophyll and related chromophores, and the energy/electron transfer processes critical to photosynthesis, to teach molecular electronic spectroscopy and kinetics from new, application-oriented perspectives. Finally, a broad, science-based lecture on general and fundamental aspects of photosynthesis with links to interests in gardening will be developed as a proposed part of an adult education outreach program at the Missouri Botanical Garden. Despite the undeniable relevance of photosynthesis, the program does not have such a lecture/education component at present. In these ways we will endeavor to disseminate an understanding of photosynthesis, its relation to the earth's habitat, and to solar energy research to a broad spectrum of individuals.