ИСТИНА

The kinetic and thermodynamic parameters of the primary photochemical reactions in the reaction center of the canonical Chl a containing photosystem I (PS I) remain under debate. The reason lies in the chemical structure of this pigment-protein complex, which contains spectrally overlapping Chl a both in antennae and reaction center (RC). Unique Chl d-based PSI from far-red light utilizing cyanobacterium A. marina contains in RC except of Chl d also two molecules of Pheo a [1, 2]. We used it as spectral marker to resolve the kinetic of the charge separation and charge stabilization reactions applying femtosecond transient absorption spectroscopy. Absorption changes were monitored in the 400–860 nm spectral window from 0.1 up to 500 ps upon unselective antenna excitation and selective excitation of the Chl d special pair P740 in the RC. Kinetic analysis made it possible to unambiguously determine the sequence of primary charge separation and charge stabilization reactions [3]. It was shown that the primary electron donor in this system is a special pair P740, and the primary acceptor is a conjugated heterodimer of Chl d2 and Pheo a3. When PS I from A. marina is excited in the near-infrared range, the formation of a primary ion-radical pair occurs with a characteristic time of 1.6 ps, while the subsequent reaction of the Pheo a3 reduction is faster than 0.2 ps and was unresolvable in the kinetics. Since P740 absorption spectrum is red-shifted by 40 nm compared to P700 in canonical PSI, its excited state has ~0.1 mV lower energy. Yet, the energy level of the stabilized ion-radical state P740(+)Pheo a3(−) was determined to be ~60 meV below that of the RC excited state, making primary reaction thermodynamically favorable in this peculiar low-energy system. [1] Hamaguchi et al., Nat. Commun. 12, 1 – 10 (2021) [2] Xu et al., J. Integr. Plant Biol. 63, 1740–1752 (2021) [3] Petrova et al., BBA – Bioenerg. 1864, 148984 (2023)