Primary charge separation in the Photosystem 2 core from Synechocystis: a comparison of femtosecond visible/mid-IR pump-probe spectra of wild type and two P680 mutants
Mariangela Di Donato 1, Rachel O. Cohen 2, Bruce A. Diner 2, Jacques Breton 3, Rienk van Grondelle 1* and Marie Louise Groot 1
1 Vrije Universiteit Amsterdam
2 du Pont de Nemours & Co
3 CEA Saclay
* To whom correspondence should be addressed. E-mail: rienk{at}nat.vu.nl.
Submitted on September 20, 2007
Revised on October 24, 2007
Accepted on 30 January 2008
 |
Abstract |
|---|
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. In order to identify spectral signatures of BA, and to further clarify the process of primary charge separation, we have compared the femtosecond-IR pump-probe spectra of the wild type 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 C9=O 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 FTIR difference spectra of the wild type (WT) and the D2-His197Ala mutant indeed show that a modification of the axially coordinating ligand to PB induces a charge redistribution of P680+. In addition, a comparison of the time-resolved vis/mid-IR spectra of the wild type 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.
Key Words:
FTIR, charge separation, energy transfer, photosynthesis, time resolved spectroscopy
