The Oxidation Potential of Chl a Is the Lowest — A New Scheme for O2 Evolution in PS II

Abstract

Oxygenic photosynthesis universally uses Chl a for P680. For water oxidation, higher oxidation potential of chlorophyll is supposed to be favorable. We found, however, Chl a had lower oxidation potential than Chls b and d. Phes a, b and d showed remarkably higher potentials than Chls. The results suggest that Phes, not Chls, might be favorable to water oxidation, although Phes has not yet been used in P680. Further, conventional scheme for water oxidation in PS II has a fundamental problem; redox potential of the Mn-complex is fixed during S-cycle. To explain the enigma, we propose a unique model for O2 evolution, where the stepwise positive shifts of oxidation potentials of the Mn-complex take place to create the great high oxidation power to oxidize water. Lower oxidation states may accept holes from P680+, but the highest oxidation state cannot do this and should utilize photon energy to attain the final state to oxidize water.