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

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Andrey A. Sukhanov¹, Georgy E. Milanovsky^2,a*, Liya A. Vitukhnovskaya^2,3, Mahir D. Mamedov², Kev M. Salikhov¹, Alexey Yu. Semenov^2,b*

¹Federal Research Center “Kazan Scientific Center of the Russian Academy of Sciences”, Zavoisky Physical-Technical Institute, 420111 Kazan, Russia

²A. N. Belozersky Research Institute of Physicochemical Biology, Lomonosov Moscow State University, 119992 Moscow, Russia

³N. N. Semenov Federal Research Center for Chemical Physics of the Russian Academy of Sciences, 119991 Moscow, Russia

^* To whom correspondence should be addressed.

Received: August 18, 2024; Revised: September 16, 2024; Accepted: September 23, 2024

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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 Synechocystis sp. 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 A branch of redox cofactors. The kinetics of the backward electron transfer from A_1B^– to P₇₀₀⁺ in the B branch of redox cofactors with τ < 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 > 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.

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KEY WORDS: photosystem I, kinetics of EPR spectra, forward and backward electron transfer, low-temperature measurements, F_X-core, A₁-core

DOI: 10.1134/S0006297924100158

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