Primary charge separation within the structurally symmetric tetrameric ChlPPChl chlorophyll exciplex in photosystem I.

In Photosystem I (PS I), the role of the accessory chlorophyll (Chl) molecules, Chl and Chl (also termed A and A), which are directly adjacent to the special pair P and fork into the A- and B-branches of electron carriers, is incompletely understood. In this work, the Chl and Chl transient absorption ΔA(λ) at a time delay of 100 fs was identified by ultrafast pump-probe spectroscopy in three pairs of PS I complexes from Synechocystis sp. PCC 6803 with residues PsaA-N600 or PsaB-N582 (which ligate Chl or Chl through a HO molecule) substituted by Met, His, and Leu. The ΔA(λ) spectra were quantified using principal component analysis, the main component of which was interpreted as a mutation-induced shift of the equilibrium between the excited state of primary donor P and the primary charge-separated state PChl. This equilibrium is shifted to the charge-separated state in wild-type PS I and to the excited P in the PS I complexes with the substituted ligands to the Chl and Chl monomers. The results can be rationalized within the framework of an adiabatic model in which the P is electronically coupled with the symmetrically arranged monomers Chl and Chl; such a structure can be considered a symmetric tetrameric exciplex ChlPPChl, in which the excited state (ChlPPChl)* is mixed with two charge-transfer states PChl and PChl. The electron redistribution between the two branches in favor of the A-branch apparently takes place in the picosecond time scale after reduction of the Chl and Chl monomers.