Analysis of light-induced vibrational structural changes of photoreceptors and antenna pigments (Sfb 498 / Heyne B11)
In this project we investigate interactions between cofactor and its protein environment by ultrafast polarization-resolved spectroscopy in the mid-infrared and visible spectral range. We mainly focus on two issues:
(a) Chromophore geometry and its interactions with the protein-binding pocket in phytochrome Structural information on the chromophore geometry of the ground state can be obtained by ultrafast polarization-resolved nonlinear infrared (IR) spectroscopy. The measured angles between the electronic transition-dipole moment and the transition-dipole moments of vibrational modes are directly connected to the chromophore geometry. Especially, the pure N-H in-plane bending marker modes (at 1570 cm-1) of the tetrapyrrole rings B and C of the chromophore can provide direct insight into the orientation of the ring planes with respect to the electronic transition-dipole moment. Analysis of chromophore and protein vibrational changes in the fingerprint region in response to chromophore excitation reveals dynamical and structural information on the chromophore and the protein-binding pocket, in the ground state, the electronic excited state, and the first product state. Combining polarization-resolved IR spectroscopy with density-functional calculations will improve knowledge on the chromophore geometry inside the phytochrome binding-pocket, resulting in a better understanding of the essential interactions for the initial reaction steps of phytochrome.
(b) Chlorophyll-protein and chlorophyll-chlorophyll interactions We study the properties of chlorophyll molecules in the ground state and the electronic excited state in different solvents, which is a prerequisite for understanding the photophysical properties of protein-bound chlorophylls. Questions on ligand binding and specific solute-solvent interactions in ground and excited states will be addressed, using ultrafast VIS-pump / IR-probe spectroscopy. The chlorophyll-protein interaction in an intact protein will be explored for the cytochrome b6f complex, which binds a single chlorophyll molecule. By probing the vibrational dynamics of the chromophore and the protein surroundings we are able to determine which amino-acid residues and other cofactors directly interact with the chlorophyll. Chlorophyll-chlorophyll interactions are of fundamental importance for excitation-energy transfer in natural photosynthesis. The main problem in photosystems is usually to isolate a subset of chlorophylls and to study their interactions. A subset of so-called "red-absorbing" chlorophylls can selectively be excited in photosystem I. We apply time-resolved Vis pump / IR probe spectroscopy to analyze the dynamics of these "red-absorbing" chlorophylls.