ABNORMAL REDOX REACTIONS IN PHOTOSYNTHETIC O2-EVOLVING CENTERS IN NACL/EDTA-WASHED PS-II - A DARK-STABLE EPR MULTILINE SIGNAL AND AN UNKNOWN POSITIVE CHARGE ACCUMULATOR

Redox events that occur in photosynthetic O2-evolving centers in NaCl/EDTA-washed PS II membranes were investigated by means of low temperature EPR and thermoluminescence. The following results have been obtained: (i) In the washed membranes, O2 centers could maintain the S2 state more than 3 h in darkness at room temperature. This dark-stable S2 was modified as seen by a multiline EPR signal with reduced hyperfine line spacing and also by a thermoluminescence band with upshifted peak temperature. (ii) This modified S2 state had an abnormally long life of t1/2 = 7 h at 20-degrees-C, and its appearance required the presence of EDTA in the medium. On addition of exogenous Ca2+, the modified S2 was converted in darkness to normal S2, and then decayed rapidly to be undetectable. (iii) On illuminating this modified S2, an EPR signal centering at aroung g = 2 was newly induced at no expense of the dark-stable EPR multiline signal. This EPR signal was accompanied by a new thermoluminescence band peaking at around 5-degrees-C, suggesting the presence of a new redox component whose oxidized form is capable of providing a positive charge for thermoluminescence in place of Mn. (iv) This new component was efficiently oxidized by illumination at -5-degrees-C but much less at -60-degrees-C, showing a half-inhibition temperature at around -40-degrees-C. (v) Addition of various divalent cations in place of Ca2+ variously affected both thermoluminescence glow peaks arising from the dark-stable S2 or from the new redox component, suggesting a cation-species-dependent modulation of the redox properties of both components. (vi) Both of these two thermoluminescence bands showed no dependency on flash number, suggesting interruption of further oxidation beyond their respective abnormal states. On addition of Ca2+, all these abnormal properties were abolished and normal period-four flash pattern was restored. These abnormal properties of the redox events in NaCl/EDTA-washed PS II membranes were discussed in relation to the demand for exogenous Ca2+ in recovery of normal properties.