Current state of the primary charge separation mechanism in photosystem I of cyanobacteria

This review analyzes new data on the mechanism of ultrafast reactions of primary charge separation in photosystem I (PS I) of cyanobacteria obtained in the last decade by methods of femtosecond absorption spectroscopy. Cyanobacterial PS I from many species harbours 96 chlorophyll a (Chl a ) molecules, including six specialized Chls denoted Chl 1A /Chl 1B (dimer P 700 , or P A P B ), Chl 2A /Chl 2B , and Chl 3A /Chl 3B arranged in two branches, which participate in electron transfer reactions. The current data indicate that the primary charge separation occurs in a symmetric exciplex, where the special pair P 700 is electronically coupled to the symmetrically located monomers Chl 2A and Chl 2B , which can be considered together as a symmetric exciplex Chl 2A P A P B Chl 2B with the mixed excited (Chl 2A P A P B Chl 2B ) * and two charge-transfer states P 700 + Chl 2A − and P 700 + Chl 2B − . The redistribution of electrons between the branches in favor of the A -branch occurs after reduction of the Chl 2A and Chl 2B monomers. The formation of charge-transfer states and the symmetry breaking mechanisms were clarified by measuring the electrochromic Stark shift of β-carotene and the absorption dynamics of PS I complexes with the genetically altered Chl 2B or Chl 2A monomers. The review gives a brief description of the main methods for analyzing data obtained using femtosecond absorption spectroscopy. The energy levels of excited and charge-transfer intermediates arising in the cyanobacterial PS I are critically analyzed.