A peculiarity of cyanobacterial Photosystem I (PSI) is the presence of so-called red chlorophylls absorbing at wavelengths longer than the reaction center P700. The origin and function of these chlorophylls have been debated in literature, but so far no consensus has been reached on either question. Here, we use plasmon-enhanced single-particle fluorescence spectroscopy to elucidate the origin of both short- and long-wavelength emitting species in monomeric PSI from Thermosynechococcus elongatus at room temperature. Polarized fluorescence spectra of single PSI complexes reveal a phase shift in the modulation of the short-wavelength (687 nm) and long-wavelength (717 nm) peaks. Numerical simulations show that this phase shift reflects a spatial angle of 15° between the transition dipole moments of the two forms. Quantum chemical calculations, together with reported X-ray structural and spectroscopic data, were used to assign the chlorophyll a monomer A3 as a candidate for the short-wavelength emitter and the B31-B32 chlorophyll dimer as a candidate for the long-wavelength emitter.

Biophysik der Photosynthese Humboldt Universität zu Berlin Philippstr 13 10115 Berlin Germany

Department of Chemical Physics and Optics Faculty of Mathematics and Physics Charles University Ke Karlovu 3 121 16 Prague Czech Republic

Department of Materials Science and Engineering Tokyo Institute of Technology Ookayama 2 12 1 S8 44 Meguro ku Tokyo 152 8552 Japan

Harris Science Research Institute Doshisha University 1 3 Miyakodani Tatara Kyotanabe Kyoto 611 0394 Japan

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