Primary Electron Transfer Processes in Photosynthetic Bacterial Reaction Centers

Washington University, Saint Louis MO

Investigators

Abstract

Intellectual Merit: The objective of this research is to achieve a unified molecular-level understanding of how the energy of sunlight is captured and stored via the primary electron transfer reactions in the bacterial photosynthetic reaction center (RC). The RC from photosynthetic bacteria is a protein-pigment complex housing two separate but quasi-symmetric branches of cofactors (bacteriochlorophyll, derivatives of it, and quinones). In principle, either branch of pigment cofactors should be able to transport electrons. Yet in the native "wild-type" bacterial RC only the "L-branch" pigments are utilized for primary photo-induced charge separation, while the alternative, "M-branch?, cofactors are completely inactive. During the course of the last ten years of research, electron transfer along the chain of M-branch cofactors was achieved albeit in much less than the 100% yield attained by the native L-branch. Trapping the first charge-separated state that forms on the L-branch was also achieved. This state lives less than a trillionth of a second and has only a small transient population during the normal course of native L-branch electron transfer. These advances have opened the door to a comprehensive molecular-level understanding of the origins of the unidirectional L-branch charge separation in the native RC and analogous understanding of electron transfer along the normally inactive M-branch of RC cofactors. Studies that seek these twin goals are the basis of this project. The goals will be pursued via state-of-the art time-resolved spectroscopic investigations spanning the sub-picosecond time scale (less than a trillionth of a second) to seconds. Sophisticated data analysis and computer modeling will be undertaken to compare results from various mutants with each other and the native RC. Specific interactions between the pigments and surrounding protein residues will be targeted for investigation to determine whether and how such interactions fine-tune cofactor properties and thus may serve to "switch" between initial L-side versus M-side electron transfer. The studies will also probe the contribution of electronic couplings between the pigments (as distinguished from issues of energetics) in controlling directionality. The combined findings will help to elucidate the mechanistic underpinnings of electron transfer in the bacterial RC as a unified whole. Broader Impact The studies pursued in this project are relevant to understanding electron transfer in the two photosystems of plants and charge migration in membrane-bound proteins in general. Understanding the molecular-level mechanisms of photo-induced charge separation in the photosynthetic RC has fundamental and far-reaching direct impacts on synthetic systems for solar-energy harvesting/conversion (including avenues being explored by the Co-PIs), thereby addressing a national need for next-generation renewable energy sources. This project will continue to have a demonstrated positive impact on the participation of undergraduates and underrepresented groups in research and science, and in the broad multidisciplinary training of students. The integration of research ideas into teaching and educational development and dissemination to the broader community will continue to be a focus. Such projects include the following: (1) A web-based tutorial will be further developed that can be tailored for undergraduate or high school students on the topic "Why is grass green and blood red?" (2) Two undergraduate physical-chemistry laboratory experiments 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 application-oriented perspectives. In one such direct application, students will design, build, and characterize a simple solar cell. (3) A broad, science-based talk on general and fundamental aspects of photosynthesis with links to interests in gardening, global climate change, and solar energy will be developed as a program for the St. Louis Master Gardener Speaker's Bureau at the Missouri Botanical Garden. Additionally, local and national popular print and web-based publication venues will continue to be exploited through contacts with science writers in order to disseminate the goals, relevance, and applications of photosynthesis research to a broad audience. This project is receiving co-funding from the Chemistry of Life Processes program in the Chemistry Division