Specificity of Photochemical Energy Conversion in Photosystem I from the Green Microalga Chlorella ohadii

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Dmitry A. Cherepanov^1,2,a*, Anastasiya A. Petrova², Mariya S. Fadeeva³, Fedor E. Gostev¹, Ivan V. Shelaev¹, Victor A. Nadtochenko^1,4, Alexey Yu. Semenov^1,2,b*

¹Semenov Federal Research Center for Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russia

²Belozersky Research Institute of Physico-Chemical Biology, Lomonosov Moscow State University, 119992 Moscow, Russia

³The George S. Wise Faculty of Life Sciences, Tel Aviv University, 69978 Tel Aviv, Israel

⁴Faculty of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia

^* To whom correspondence should be addressed.

Received: March 30, 2024; Revised: April 23, 2024; Accepted: April 24, 2024

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Primary excitation energy transfer and charge separation in photosystem I (PSI) from the extremophile desert green alga Chlorella ohadii grown in low light were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850 nm and in the time range from 50 fs to 500 ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band and compared with similar processes in PSI from the cyanobacterium Synechocystis sp. PCC 6803. When PSI from C. ohadii was excited at 660 nm, the processes of energy redistribution in the light-harvesting antenna complex were observed within a time interval of up to 25 ps, while formation of the stable radical ion pair P₇₀₀⁺A₁⁻ was kinetically heterogeneous with characteristic times of 25 and 120 ps. When PSI was excited into the red edge of the Qy band at 715 nm, primary charge separation reactions occurred within the time range of 7 ps in half of the complexes. In the remaining complexes, formation of the radical ion pair P₇₀₀⁺A₁⁻ was limited by the energy transfer and occurred with a characteristic time of 70 ps. Similar photochemical reactions in PSI from Synechocystis 6803 were significantly faster: upon excitation at 680 nm, formation of the primary radical ion pairs occurred with a time of 3 ps in ~30% complexes. Excitation at 720 nm resulted in kinetically unresolvable ultrafast primary charge separation in 50% complexes, and subsequent formation of P₇₀₀⁺A₁⁻ was observed within 25 ps. The photodynamics of PSI from C. ohadii was noticeably similar to the excitation energy transfer and charge separation in PSI from the microalga Chlamydomonas reinhardtii; however, the dynamics of energy transfer in C. ohadii PSI also included slower components.

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KEY WORDS: green alga Chlorella ohadii, femtosecond laser spectroscopy, primary photochemical reactions, exciton dynamics, charge separation

DOI: 10.1134/S0006297924060129

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