MIDPOINT POTENTIAL OF SIGNAL-II (SLOW) IN TRIS-WASHED PHOTOSYSTEM-II PARTICLES

In Tris-washed photosystem-II particles [from pea chloroplasts] an EPR signal was induced in the dark by addition of an Ir salt (K2IrCl6). This signal is attributed to signal IIs (slow) (D+) and the redox titration gives an Em [mid-point potential] value of 760 mV for the couple D+/D. On the basis of previous studies on the equilibrium between D+Z and DZ+ (K = 104) (Boussac and Etienne, 1982) a value of 1 V is attributed for the Em of the Z+/Z couple. A 2nd effect of K2IrCl6 is to modify the spectral characteristics of signal II. K2IrCl6 is apparently able to change the environment of the species from which signal IIs and signal IIf originate.