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Excitation-energy-transfer processes in pigment-protein complexes in photosynthetic organisms are often changed under different pH conditions. However, it is unclear how the pH changes affect excitation-energy relaxations in photosystem I (PSI) cores. In this study, we examined the pH sensitivity of energy dynamics in PSI tetramer, dimer, and monomer isolated from a cyanobacterium Anabaena sp. PCC 7120 by means of time-resolved fluorescence spectroscopy. Each PSI was adapted to pH 5.0, 6.5, and 8.0. Fluorescence decay-associated (FDA) spectra of the pH-8.0 PSI dimer and monomer showed positive and negative peaks within 5 ps, whereas the FDA spectra of the PSI tetramer did not show such 5-ps fluorescence component. Mean lifetimes of the fluorescence at pH 6.5 are shorter in the PSI tetramer than in the PSI dimer and monomer, indicating an accelerated energy quenching in the tetramer. The effects of the acidic and basic pHs on the energy-transfer processes differ significantly among the three types of PSI, suggesting different pH-sensing sites around pigment molecules in the three PSIs. Based on these results, together with our recent structural finding of the PSI tetramer, we discuss functional implications for the pH-sensing regulation of the excitation-energy transfer in the PSI tetramer.
This article was published in the following journal.
Name: The journal of physical chemistry. B
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A large multisubunit protein complex that is found in the THYLAKOID MEMBRANE. It uses light energy derived from LIGHT-HARVESTING PROTEIN COMPLEXES to drive electron transfer reactions that result in either the reduction of NADP to NADPH or the transport of PROTONS across the membrane.
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