Elucidation of the structure-guided photophysical tuning in retinal-based phototrophic assemblies

• Applicant: Dr. Marvin Asido
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 564394927

The direct conversion of light into chemical energy was one of the key processes in the development of life on earth and is now equally important to reconcile economic growth and global sustainability in modern society. Many biological systems found in nature have evolutionarily optimized solar energy conversion - in many cases to unmatched efficiencies - by combining one or several light-absorbing cofactor/s with an apoprotein of defined function. One of the evolutionary earliest examples of solar energy conversion on earth is done by microbial rhodopsins, which are responsible for large parts of light-to-chemical-energy conversion in oceanic and freshwater systems. On the cellular level, these membrane-embedded proteins generate a proton gradient which in turn is utilized to drive ATP and NADH production. Notably, the rate of this charge transport is significantly influenced by a pH-dependent switch between monomeric and oligomeric forms of the apoproteins. While functional assays have established that oligomerization impacts transport, the underlying photochemical role remains unclear. There are two major open questions: (i) Cofactor photochemistry depends sensitively on the structure of the surrounding apoprotein. How does the structural rearrangement upon oligomerization impact the early photocycle of the primary cofactor, retinal? (ii) It was recently discovered that additional cofactors, carotenoids, are bound in broader range of rhodopsins than previously anticipated. Are there photophysical and/or photochemical pathways between the cofactors that emerge upon oligomerization? Addressing these mechanistic questions has been limited by the difficulties inherent to preparing well-defined oligomeric states for spectroscopic measurements of the rhodopsins. Here, the applicant proposes to use model membrane discs, 'nanodiscs', that recreate near-native membranes with rhodopsins embedded in defined monomeric or oligomeric forms. Furthermore, the ultrafast timescales and congested energetic landscape of the rhodopsin primary reaction typically obfuscates the photophysics and photochemical pathways. The applicant will use 2D electronic spectroscopy, a state-of-the-art method with enhanced temporal and spectral resolution, to resolve these pathways and map out cofactor couplings in the respective constructs. Through this combination, it will be possible to link the structural integrity of the protein (monomer vs oligomer) and the earliest steps of energy conversion, ultimately elucidating how microbial rhodopsins ensure a robust light-to-chemical energy conversion in a dynamically changing environment.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professorin Dr. Gabriela S. Schlau-Cohen