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

Using high-frequency pulsed EPR spectroscopy in the Q range at cryogenic temperatures, the kinetics of redox transformations of the primary electron donor ${\mathrm{Р}}_{700}^{+}$ and the quinone acceptor ${\mathrm{А}}_1^{-}$ in various complexes of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803 were simultaneously studied for the first time in the time range of 200 μs-10 ms. In the A₁-core complexes of PSI that lack 4Fe4S clusters, the kinetics of the ${\mathrm{А}}_1^{-}$ and ${\mathrm{Р}}_{700}^{+}$ signals decay at a temperature of 100 K coincided and had a characteristic time of τ ≈ 500 μs, caused by charge recombination in the ${\mathrm{Р}}_{700}^{+}$${\mathrm{А}}_1^{-}$_A ion-radical pair in the A branch of redox cofactors. The kinetics of the reverse electron transfer from ${\mathrm{А}}_1^{-}$_B to ${\mathrm{Р}}_{700}^{+}$ in the B branch of redox cofactors with τ < 100 μs could not be recorded due to the time limitations of the method. In the native PSI complexes comprising a full set of redox cofactors and in the F_X-core complexes with the 4Fe4S cluster F_X only the kinetics of the ${\mathrm{А}}_1^{-}$ signal was significantly faster than that of the ${\mathrm{Р}}_{700}^{+}$ signal. The disappearance of the ${\mathrm{А}}_1^{-}$ signal had a characteristic time of 280-350 μs. It was suggested that, in addition to the reverse electron transfer from ${\mathrm{А}}_1^{-}$_A to ${\mathrm{Р}}_{700}^{+}$ with τ ≈ 500 μs, it also includes a slowed down (up to 150-200 μs) forward electron transfer from ${\mathrm{А}}_1^{-}$_A to the 4Fe4S cluster F_X. In the kinetics of ${\mathrm{Р}}_{700}^{+}$ reduction, it was possible to distinguish components caused by the reverse electron transfer from ${\mathrm{А}}_1^{-}$ (τ ≈ 500 μs) and from 4Fe4S clusters (τ = 1 ms for the F_X-core and τ > 5 ms for native complexes). These results are in qualitative agreement with the data on the kinetics of ${\mathrm{Р}}_{700}^{+}$ reduction obtained earlier using pulsed absorption spectrometry at cryogenic temperatures.