Comparison of tryptophan fluorescence lifetimes in cyanobacterial photosystem I frozen in the light and in the dark.

The dependence on temperature of tryptophan fluorescence lifetime in trimeric photosystem I (PSI) complexes from cyanobacteria Synechocystis sp. PCC 6803 during the heating of pre-frozen to - 180 °C in the dark or in the light-activated preparations has been studied. Fluorescence lifetime in samples frozen in the light was longer than in samples frozen in the dark. For samples in 65% glycerol at λreg = 335 nm and at 20 °C, the lifetime of components were as follows: τ1 ≈ 1.2 ns, τ2 ≈ 4.9 ns, and τ3 ≈ 20 ns. The contribution of the first component was negligible. To analyze the contribution of components 2 and 3 derived from frozen-thawed samples, two temperature ranges from - 180 to - 90 °C and above - 90 °C are considered. In doing so, the contributions of these components appear antiphase course to each other. The dependence on temperature of these contributions is explained by the influence of the microconformational protein dynamics on the tryptophan fluorescence lifetime. In the present work, a comparative analysis of temperature-dependent conformational dynamics and electron transfer in cyanobacterial PSI (Schlodder et al., in Biochemistry 37:9466-9476, 1998) and Rhodobacter sphaeroides reaction center complexes (Knox et al., in J Photochem Photobiol B 180:140-148, 2018) was also carried out.