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

The kinetics of the primary electron donor P₇₀₀⁺ and the quinone acceptor A₁^- 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 <i>Synechocystis sp.</i> PCC 6803. In the A₁-core PSI complexes that lack 4Fe4S clusters, the kinetics of the A₁^- and P₇₀₀⁺ signals disappearance at 100 K were similar and had a characteristic time of τ ≈ 500 μs, caused by charge recombination in the P₇₀₀⁺A_1A^- ion-radical pair in the <i>A</i> branch of redox cofactors. The kinetics of the backward electron transfer from A_1B^- to P₇₀₀⁺ in the <i>B</i> branch of redox cofactors with τ &lt; 100 μs could not be resolved due to time limitations of the method. In the native PSI complexes with a full set of redox cofactors and in the F_X-core complexes, containing the 4Fe4S cluster F_X, the kinetics of the A₁^- signal was significantly faster than that of the P₇₀₀⁺ signal. The disappearance of the A₁^- signal had a characteristic time of 280-350 μs; it was suggested that, in addition to the backward electron transfer from A_1A^- to P₇₀₀⁺ with τ ≈ 500 μs, its kinetics also includes the forward electron transfer from A_1A^- to the 4Fe4S cluster F_X, which had slowed down to 150-200 μs. In the kinetics of P₇₀₀⁺ reduction, it was possible to distinguish components caused by the backward electron transfer from A₁^- (τ ≈ 500 μs) and from 4Fe4S clusters (τ = 1 ms for the F_X-core and τ &gt; 5 ms for native complexes). These results are in qualitative agreement with the data on the kinetics of P₇₀₀⁺ reduction obtained previously using pulse absorption spectrometry at cryogenic temperatures.