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 B and not from the special pair P. To identify spectral signatures of B, 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 P (D2-His) 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 C9=O carbonyl of the putative primary donor B through a water molecule. The constructed mutations are thus expected to perturb the vibrational properties of B 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 P: Analysis of steady-state light-induced Fourier transform infrared difference spectra of the WT and the D2-HisAla mutant indeed shows that a modification of the axially coordinating ligand to P induces a charge redistribution of P : 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 B. © 2008 by the Biophysical Society.