Trapping kinetics in isolated cyanobacterial PS I complexes

The excitation energy trapping and the role of the 'red' chlorophyll (Chl) states in the photosystem (PS) I monomers and trimers from the cyanobacterium Thermosynechococcus elongatus were studied. We demonstrate the adequacy of the “charge recombination” model for the trapping kinetics. On the basis of this model the reaction center excited state can be resolved. The overall kinetics is shown to be trap-limited even though the presence of the 'red' Chls induces a substantial slowing down (∼60%) of the trapping. Two kinetically different 'red' Chl pools were resolved. Both of these 'red' pools originate from the same groups of pigments in either of the two aggregation states. This indicates that careful isolation does not disturb substantially the 'red' Chls and we can exclude their location at the monomer-monomer interface. Acceleration of the secondary electron transfer step in the studied complexes as compared to PS I from mesophilic organisms is observed.