Fig. 3.

Redox titrations of HiPIP II in membrane fragments and in the soluble form. EPR titration on membrane fragments (open squares) was performed in the dark in 50 mM Mops, 130 g/liter NaCl (pH 7). The amplitude of the g = 2.10 peak measured on EPR spectra recorded as in Fig. 1 is plotted versus the ambient redox potential. For this particular experiment, the data points are fitted by an n = 1 Nernst curve with E_m =+125 mV. Optical redox titrations of purified HiPIP II were performed in 50 mM Mops (pH 7) in the absence (closed squares) or the presence (open circles) of 2.5 M NaCl. The A_{475 nm} - A_{600 nm} difference is plotted versus the ambient redox potential. The best fit in the absence of NaCl is obtained with E_m =+50 mV (n = 1). In the presence of NaCl, two equivalent components with n = 1 and E_m = +50 mV and +160 mV were required.