IDENTIFICATION OF FOURIER-TRANSFORM INFRARED SIGNALS FROM THE NONHEME IRON IN PHOTOSYSTEM-II

Fourier transfer infrared (FTIR) signals originating from a high-potential electron acceptor in PS II were studied by flash-induced FTIR difference spectroscopy, Redox titration of these signals using ferri-ferrocyanide mixtures showed midpoint potentials (E(m)) of 489 +/- 12 and 426 +/- 9 mV at pH 5.5 and 6.5, respectively, revealing a pH dependence of about -60 mV/pH unit. These E(m) values and pH dependence were in good agreement with those of the non-heme iron, so-called Q(400), located between Q(A), and Q(B). This indicates that the observed FTIR signals are due to changes in ligands of the non-heme iron and surrounding protein moieties induced on photoreduction from Fe3+ to Fe2+.