Electrochemistry of Chlorophylls - New Scheme for O2 Evolution in PS II

  • Masami Kobayashi, Institute of Materials Science, University of Tsukuba, Japan
  • Mr Shunsuke Ohashi, Institute of Materials Science, University of Tsukuba, Japan
  • Mr Masataka Nakazato, Chlorophyll Research Institute, Japan
  • Mr Tatsuya Iemura, Institute of Materials Science, University of Tsukuba, Japan
  • Mr Masahiro Kasahara, Institute of Materials Science, University of Tsukuba, Japan
  • Prof Tadashi Watanabe, Institute of Industrial Science, University of Tokyo, Japan

    • In 1996, a Chl d-dominated cyanobacterium Acaryochloris marina was discovered. In the PS I reaction center of A. marina, Chl d' functions as the primary electron donor P740: a heterodimer of Chl d/d', like Chl a/a' for P700, and Chl a as the primary electron acceptor. The primary electron acceptor of PS II in A. marina has been defined as Phe a, however, whether Chl d acts as the special pair in PS II is a matter of controversy. The oxidation potential of Chl d was found to be +0.88 V vs. SHE in acetonitrile, which was higher than that of Chl a (+0.81 V), and lower than that of Chl b (+0.94 V). Corresponding pheophytins showed significantly high values around +1.2 V. For water oxidation, very high oxidation power is believed to be needed, but oxidation power of Chl a is the lowest of all Chls. To explain the enigma, we will present a unique model for O2 evolution. In our model, oxidation potential of Chl a (or d) is not high enough to oxidize water, but the stepwise positive shifts of oxidation potentials of the Mn-complex take place during the S-cycle to create the great high oxidation power to oxidize water. Lower oxidation states of the Mn-complex may accept holes from P680+, but higher oxidation state(s) cannot do this and should utilize photon energy to attain the final state to oxidize water.