Primary Charge Separation in PS2 Core Complexes from Synechocystis 6803: A Comparison of Femtosecond Visible/Mid-IR Pump-Probe Spectra of Wild Type and Two P680 Mutants

Primary charge separation in PS2 core complexes from Synechocystis sp. PCC 6803 has been studied by femtosecond visible/mid-IR pumpprobe spectroscopy upon excitation at 680 nm. It is now quite well accepted that primary charge separation predominantly starts from the accessory chlorophyll BA. In order to identify spectral signatures of BA, and to better clarify the process of primary charge separation, we have compared the femto-IR pump-probe spectra of the wild type PS2 core with those of two mutants, in which the histidine residue coordinated to PB (D2-H197) has been changed to alanine or glutamine. The mutated histidine is indirectly H-bonded to BA, through a water molecule, and the mutation is expected both to perturb the vibrational properties of BA, possibly by displacing the H-bonded water molecule, and to modify the electronic properties and the charge localization on P680 +. The comparison of the time resolved spectra of wild type and mutants allowed us to propose a band assignment identifying the characteristic vibrations of BA.