Primary Charge Separation in the Photosystem II Core from Synechocystis: A Comparison of Femtosecond Visible/Midinfrared Pump-Probe Spectra of Wild-Type and Two P680 Mutants

It is now quite well accepted that charge separation in PS2 reaction centers starts predominantly from the accessory chlorophyll BA and not from the special pair P680. To identify spectral signatures of BA, and to further clarify the process of primary charge separation, we compared the femtosecond-infrared pump-probe spectra of the wild-type (WT) PS2 core complex from the cyanobacterium Synechocystis sp. PCC 6803 with those of two mutants in which the histidine residue axially coordinated to PB (D2-His197) has been changed to Ala or Gln. By analogy with the structure of purple bacterial reaction centers, the mutated histidine is proposed to be indirectly H-bonded to the C9O carbonyl of the putative primary donor BA through a water molecule. The constructed mutations are thus expected to perturb the vibrational properties of BA by modifying the hydrogen bond strength, possibly by displacing the H-bonded water molecule, and to modify the electronic properties and the charge localization of the oxidized donor P680+. Analysis of steady-state light-induced Fourier transform infrared difference spectra of the WT and the D2-His197Ala mutant indeed shows that a modification of the axially coordinating ligand to PB induces a charge redistribution of P680+. In addition, a comparison of the time-resolved visible/midinfrared spectra of the WT and mutants has allowed us to investigate the changes in the kinetics of primary charge separation induced by the mutations and to propose a band assignment identifying the characteristic vibrations of BA.