In all photosynthetic systems, the primary energy transduction event is the light-induced generation of a charge-separated state in a pigment-protein complex. Although the specific pathways vary among organisms, the general pattern is remarkably conserved, and an understanding of these processes serves as the basis for current designs of artificial systems that mimic the photosynthetic process. The usefulness of the reaction center from Rhodobacter sphaeroides as a model for photosystem II is illustrated by previous work in which a redox-active Mn cofactor was bound to the reaction center at a location analogous to that of the Mn cluster of photosystem II. The first specific aim of the project is to characterize the properties of the bound Mn and other new metal cofactors in the reaction center. A wide variety of spectroscopic techniques will be utilized to determine the structural properties, the electronic structure of the metal cofactor, and the electron transfer properties, providing the basis for understanding the specificity for metal binding, the factors that control the activity of the metal, and how reactions involving metal cofactors are controlled by protein interactions. The second specific aim is to design proteins with multinuclear Mn clusters. The assembly of clusters will be approached by three independent avenues: a semi-synthetic method in which a variety of Mn compounds will be bound to the protein, the application of the self-assembly principles of the Mn cluster of photosystem II, and the introduction of new ligands for the binding of binuclear and tetranuclear Mn clusters. The incorporation of new Mn cofactors should shed light into the molecular requirements for the binding of metal clusters to proteins. The insight gained regarding electron transfer proteins with new metal cofactors should provide the foundation for the design of proteins capable of light-driven metal-based catalysis. Achievement of the research goals should have an impact on understanding photosynthesis and the development of solar-energy devices.
Broader impacts The project provides multidisciplinary training for undergraduate and graduate students and international exposure through collaborations. Many project concepts, notably the role of photosynthesis in energy utilization and the fundamentals of spectroscopic techniques used to study reaction centers, will be included in the development of courses for students of different backgrounds, as well as a textbook for undergraduate education authored by the principal investigator.
Intellectual merit: In all photosynthetic systems, the primary energy transduction event is the light-induced generation of a charge-separated state in a pigment-protein complex. Although the specific pathways vary among organisms, the general pattern is remarkably conserved, and an understanding of these processes serves as the basis for current designs of artificial systems that mimic the photosynthetic process. The reaction center from Rhodobacter sphaeroides has been used by our group as a model system to incorporate features of photosystem II such as a highly oxidizing primary donor. The influence of the three-dimensional arrangement of the cofactors, the energetics of each cofactor, and the dynamics of the protein on the electron transfer properties of bacterial reaction centers was investigated. The ability of photosynthetic complexes to efficiently transfer energy was examined by determining the three-dimensional structure of the FMO antenna complex, examining the role of hydrogen bonds, and using femtosecond optical spectroscopy to probe the characteristics of the carotenoid in the reaction center. The inclusion of an Mn binding site in the reaction center with a high midpoint potential resulted in oxidation of a bound Mn atom located in a position analogous to that of photosystem II. A key advance is the addition of a redox active mononuclear Mn cofactor, providing the opportunity to study novel manganese cofactors and to introduce new light-driven reactions into this photosystem. Overall, the molecular insight gained from these studies should have a general impact on developing design principles for metalloenzymes, understanding electron transfer mechanisms in photosynthesis, and improving energy conversion technologies. Broader impacts: Our research and educational activities promote progress in science by providing wider opportunities for participation in undergraduate research, meeting challenges in undergraduate education by writing textbooks, creating a curriculum for a new interdisciplinary graduate program, and fostering public communication and outreach by students.
