Fluorescence of the various red antenna states in photosystem I complexes from cyanobacteria is affected differently by the redox state of P700

Photosystem I of cyanobacteria contains different spectral pools of chlorophylls called red or long-wavelength chlorophylls that absorb at longer wavelengths than the primary electron donor P700. We measured the fluorescence spectra at the ensemble and the single-molecule level at low temperatures in the presence of oxidized and reduced P700. In accordance with the literature, it was observed that the fluorescence is quenched by P700+. However, the efficiency of the fluorescence quenching by oxidized P700+ was found to be extremely different for the various red states in PS I from different cyanobacteria. The emission of the longest-wavelength absorbing antenna state in PS I trimers from Thermosynechococcus elongatus (absorption maximum at 5 K: ≅ 719 nm; emission maximum at 5 K: ≅ 740 nm) was found to be strongly quenched by P700+ similar to the reddest state in PS I trimers from Arthrospira platensis emitting at 760 nm at 5 K. The fluorescence of these red states is diminished by more than a factor of 10 in the presence of oxidized P700. For the first time, the emission of the reddest states in A. platensis and T. elongatus has been monitored using single-molecule fluorescence techniques.

► Quenching by P700+. ► Red/reddest antenna states. ► Reassignment of absorption bands.