Kinetics of Electron Transfer between Redox Cofactors in Photosystem I Measured by High-Frequency EPR Spectroscopy.

The kinetics of the primary electron donor P and the quinone acceptor A redox transitions were simultaneously studied for the first time in the time range of 200 μs-10 ms using high-frequency pulse Q-band EPR spectroscopy at cryogenic temperatures in various complexes of photosystem I (PSI) from the cyanobacterium PCC 6803. In the A-core PSI complexes that lack 4Fe4S clusters, the kinetics of the A and P signals disappearance at 100 K were similar and had a characteristic time of τ ≈ 500 μs, caused by charge recombination in the PA ion-radical pair in the branch of redox cofactors. The kinetics of the backward electron transfer from A to P in the branch of redox cofactors with τ < 100 μs could not be resolved due to time limitations of the method.

Changes in the Electron Transfer Symmetry in the Photosystem I Reaction Centers upon Removal of Iron-Sulfur Clusters.

In photosynthetic reaction centers of intact photosystem I (PSI) complexes from cyanobacteria, electron transfer at room temperature occurs along two symmetrical branches of redox cofactors A and B at a ratio of ~3 : 1 in favor of branch A. Previously, this has been indirectly demonstrated using pulsed absorption spectroscopy and more directly by measuring the decay modulation frequencies of electron spin echo signals (electron spin echo envelope modulation, ESEEM), which allows to determine the distance between the separated charges of the primary electron donor P and phylloquinone acceptors A and A in the symmetric redox cofactors branches A and B. In the present work, these distances were determined using ESEEM in PSI complexes lacking three 4Fe-4S clusters, F, F, and F, and the PsaC protein subunit (the so-called P-A core), in which phylloquinone molecules A and A serve as the terminal electron acceptors.

Autobiography.

On the occasion of my 85th birthday, I share insights about my life and science. This autobiography has been written in an interesting period of my life. Two years ago I formulated a new paradigm in a particular scientific discipline: spin exchange.

Unusually large hyperfine structure of the electron spin levels in an endohedral dimetallofullerene and its spin coherent properties.

We report the synthesis, ESR spectroscopic and spin coherent properties of the dimetallofullerene Sc2@C80(CH2Ph). The single-electron metal-metal bond of the Sc2 dimer inside the fullerene's cage is stabilized with the electron spin density being fully localized at the metal bond. This results in an extraordinary strong hyperfine interaction of the electron spin with the 45Sc nuclear spins with a coupling constant a = 18.

Interpretation of the Nature of the Mixed Form of Resonance Lines of the Electron Paramagnetic Resonance Spectrum in a New Paradigm of Spin Exchange: Abnormal "Resonance" of Non-Resonant Spins.

It is shown that the asymmetric shape of lines in the electron paramagnetic resonance spectra of paramagnetic particle solutions caused by spin exchange in bimolecular particle collisions is due to the fact that, in the experiment, under the conditions of slow spin exchange, an absorption signal of nominally resonant spins and a "resonance" dispersion signal of nominally nonresonant spins with an abnormal phase of their quantum coherence are registered simultaneously. The mixing of these two signals is different for different resonance lines in the observed spectrum. A phenomenological model is proposed that can be used to find the phase of the "resonance" signal of non-resonant spins.