Reinvestigation on primary processes of PSII-dimer from Thermosynechococcus vulcanus by femtosecond pump-probe spectroscopy

Cyanobacterial photosynthetic apparatus efficiently capture sunlight, and the energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. PSII is a unique membrane protein complex that photo-catalyzes oxidation of water and majorly contains photosynthetic pigments of chlorophyll a and carotenoids. In the present study, the ultrafast energy transfer and charge separation dynamics of PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus were reinvestigated by femtosecond pump-probe spectroscopic measurements under low temperature and weak intensity excitation condition. The results imply the two possible models of the energy transfers and subsequent charge separation in PSII. One is the previously suggested “transfer-to-trapped limit” model. Another model suggests that the energy transfers from core CP43 and CP47 antennas to the primary electron donor Chl_D1 with time-constants of 0.71 ps and 3.28 ps at 140 K (0.17 and 1.33 ps at 296 K), respectively and that the pheophytin anion (Pheo_D1⁻) is generated with the time-constant of 43.0 ps at 140 K (14.8 ps at 296 K) upon excitation into the Q_y band of chlorophyll a at 670 nm. The secondary electron transfer to quinone Q_A: Pheo_D1⁻Q_A → Pheo_D1Q_A⁻ is observed with the time-constant of 650 ps only at 296 K. On the other hand, an inefficient β-carotene → chlorophyll a energy transfer (33%) occurred after excitation to the S₂ state of β-carotene at 500 nm. Instead, the carotenoid triplet state appeared in an ultrafast timescale after excitation at 500 nm.