Study of Electron Transfer in Photosystem I Using High-Frequency EPR Spectroscopy. In Memory of Professor Klaus...

Klaus Möbius, Professor at the Free University of Berlin, was an outstanding physical chemist and biophysicist. He was a pioneer in the development of high-field/high-frequency EPR spectroscopy methods and their application in the study of photosynthesis. Among the most essential are the applications in studying the charge transfer kinetics and properties of the ion-radical pairs in photosynthetic reaction centers (RC). Under his leadership and with his direct participation a unique setup allowing registration of the kinetics of the electron transfer between the (bacterio)chlorophyll dimer and quinone in the bacterial photosynthetic RC and plant photosystem I (PSI) was created. This setup also allowed precise determining of the distance between separated charges based on measuring the frequencies of the Electron Spin Echo Envelope Modulation (ESEEM). This setup made it possible to prove that electron transfer in PSI occurs mainly along the A branch of redox cofactors. The kinetics of backward electron transfer reaction (reoxidation of the phyllosemiquinone anion A₁^- and reduction of the photooxidized chlorophyll dimer P₇₀₀⁺) in PSI were measured under the same conditions. The essential data on the bioprotective effect of the disaccharide trehalose on the kinetics of forward and backward electron transfer in PSI complexes were obtained. A significant slowdown in the kinetics of electron transfer due to the restriction of protein conformational mobility, as well as long-term maintaining of functional activity of PSI dried in a vitreous trehalose matrix at room temperature (i.e., subjected to a reversible anhydrobiosis) was demonstrated. These results obtained in collaboration with Prof. Möbius and Prof. Venturoli (Bologna) allowed elucidating the role of hydrogen bond network and the conformational mobility of the protein subunits in facilitating electron transfer in the photosynthetic RC.