Our biophysically oriented research activities focus on Photosynthesis. It is the most important process for Life on Earth. It provides all the oxygen we breathe, all the food we eat and all the fossil fuels we burn for our civilization. Understanding the underlying physical, chemical and biological principles of photosynthetic solar-energy conversion and sustainable storage – and ultimately copying them for “artificial” photosynthesis devices – is among the grand challenges of the cultural world under the threat of global climate change.
We are studying light-induced transport processes of electrons and ions in membrane-bound protein complexes, for example Reaction Centers from photosynthetic bacteria and plants, employing modern methods of pulsed paramagnetic electron spin resonance (EPR), electron-electron double resonance (ELDOR) and electron-nuclear double resonance (ENDOR) spectroscopy, in particular at high magnetic fields, as well as fast optical absorption spectroscopy. By combining EPR and optical spectroscopies, we cover a wide time-scale of photochemical reactions, ranging from tens of femtoseconds to tens of seconds. The EPR experiments are performed at the Max Planck Institute for Chemical Energy Conversion, Mülheim a.d. Ruhr, the optical experiments at the University of Bologna, the Moscow State University and the Pennsylvania State University.
The spectroscopic data are analyzed by state-of-the-art quantum-chemical methods to determine the electronic and geometrical structures (distances and orientations) of the molecules involved in the biological process, including their conformational changes during the reaction.
The research activities are performed in collaboration with scientists from Germany, Italy, Russia, USA, Israel, Japan, The Netherlands and Poland.
Current research topics of prime interest
For cutting-edge high-field/high-frequency EPR spectroscopy development of new stationary and pulsed EPR, ENDOR, ESEEM, ELDOR and ELDOR-detected NMR methods, e.g., at 3.5 T/95 GHz.
Elucidation of the relationship between structure, dynamics and function of donor and acceptor pigment molecules and their dynamics during electron-transfer processes in Reaction Centers of photosynthesis and their model systems. Natural energy-transducing photosynthetic systems, such as Reaction Centers of anoxygenic bacteria and Photosystem I and Photosystem II of oxygenic bacteria and green plants represent important examples for such studies.
These processes are controlled by adjusting the matrix environment (“solvent”, hydrogen-bonding networks, hydration level, temperature, pressure) of the cofactors in their protein binding sites. The electron-transfer pathway is also controlled by site-specific mutagenesis of the photosynthetic protein complex. Besides mutants with modified amino-acid positions also site-directed mutants of spin-labeled organisms are employed with attached nitroxide spin labels as electron-spin probe for EPR experiments.
The main goal of the research projects is to better understand the complex physical and chemical interactions between nanoscale protein molecules and liquid solvents or solid sugar-glass matrices in which they are embedded. These studies go hand in hand with investigations of the molecular regulation mechanisms of electron- and proton-transfer processes under extreme environmental conditions, such as dryness, heat and hydrostatic pressure. During the last few decades, sugar-glass matrices have attracted a growing interest not only in the food-storage industry but also in the biotechnology community for their ability to stabilize labile proteins, including therapeutic polypeptides, and to optimize their storage at room temperature in the solid state. Among other disaccharides, trehalose (α-D-glucopyranosyl α-D-glucopyranoside) forms, upon dehydration at room temperature, glassy matrices that protect the hosted protein against denaturation. And this without the cellular damages that are typically induced by conventional freezing, heating, and drying protocols. In nature, the bioprotective action of trehalose and other disaccharide glasses is used by several “anhydrobiotic” organisms, allowing survival for long periods (up to years) of extreme draught and high temperatures, as prevail in many deserts, by preserving the integrity of their cellular structures, while reversibly arresting their metabolism (“anhydrobiosis” means “life without water”) until the next rain comes.
Spectroscopic data analysis
State-of-the-art quantum-chemical calculations in the frame of advanced (ORCA) Density Functional Theory (DFT) to translate spectroscopic parameters into molecular parameters of structure and dynamics. Characteristic shifts of g-, hyperfine- and quadrupole-tensor components are interpreted in terms of changes in the geometric and electronic molecular structure, explicitly including hydrogen-bond effects. Of particular interest are site-directed NO spin labels in their protein environment as well as cofactor ion radicals and radical pairs in wild-type and mutant reaction centers.
In addition to quantum-chemical calculations the analysis of the data is performed by methods of statistical physics and molecular dynamics. The results of molecular dynamics simulations, and their examination within modern theories of charge transfer in heterogeneous media, will provide a deeper insight into the mechanism of efficient conversion and storage of light energy in photosynthetic protein complexes. The results will be of relevance both in basic science and technological applications in the topical area of artificial photosynthesis.