Redox potential of plastoquinone A in spinach chloroplasts.

The redox potential of plastoquinone A in spinach chloroplasts was determined. The midpoint potential of the quinone is about +80 mV at pH 7.0 with an n value of 2. The pH-dependence of the potential is -30 mV per pH between pH 4.0 and 5.7, and -60 mV per pH between pH 5.7 and 8.0. The change of the slope at pH 5.7 is interpreted as the protonation of the oxidized plastoquinone A.