Shedding new light on photosynthesis: Advanced techniques of two-dimensional spectroscopy

• Applicant: Dr. Julian Lüttig
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545291444

Photosynthetic systems utilize a complex network of pigments and proteins to harvest light. The protein-pigment complexes form various structures in which the protein scaffold fixes the relative orientations of the pigments, controlling their coupling and leading to the formation of delocalized excited states, i.e., excitons. Exciton transport occurs on an ultrafast timescale from the antenna complexes to the reaction center (RC) where charge separation takes place. The distinct structures of antenna and RC employed by different photosynthetic organisms reflect the high degree to which they have adapted to their environment. Photosynthetic systems also adapt dynamically to changing environments by altering the structure and stoichiometry of antenna and RC complexes. How can we observe the ultrafast exciton dynamics throughout the photosynthetic system and identify general design principles? Two-dimensional (2D) spectroscopy utilizes a series of ultrashort laser pulses to track ultrafast dynamics. We will use several variants of 2D spectroscopy to investigate photosynthetic systems - from antenna complexes to full cells. Each variant of 2D spectroscopy is sensitive to a specific property and using several methods in parallel allows us to obtain a comprehensive picture of photosynthesis. Broadband 2D spectroscopy maps energy transfer pathways, while 2D electronic-vibrational spectroscopy probes excitonic coupling. We will also develop two new variants of fluorescence-detected 2D spectroscopy extending the toolkit of spectroscopic methods allowing us to determine exciton delocalization and the response of subsystems with high sensitivity. To gain broad insight into the design principles of photosynthesis we will examine a variety of samples from different photosynthetic species. First, we will investigate recently discovered species of the phylum Gemmatimonadetes, in which we will characterize the excitonic properties and energy transfer and probe the mixing of vibrational and electronic degrees of freedom. We will study both isolated complexes and whole cells to probe the influence of the native membrane environment and intercomplex interactions. Second, we will study two RC mutants of purple bacteria with distinct charge transfer mechanisms. We aim to combine our measurements with simulations, achieving a model of the excitonic states and their mixing with charge transfer states. We will also characterize the role of vibronic mixing and coherence transfers. Finally, we will investigate how photosystem II adapts to far-red light, wherein the incorporation of modified chlorophylls extends the absorption spectrum into the infrared. We aim to probe how the altered composition affects exciton delocalization, transfer, and vibronic mixing. In total, we will achieve a holistic picture of excitons in photosynthesis, extracting general design principles and furthering our understanding of the structure-function relationship in photosynthetic systems.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professorin Jennifer Ogilvie, Ph.D.