Studies on the deconvolution of flash-induced absorption changes into the difference spectra of individual redox steps within the water-oxidizing enzyme system

The possibility to determine the difference spectra _i+1j of each univalent redox step S_iS_i+1(i=0,...3) of the water-oxidizing enzyme system was analyzed by theoretical calculations and by measurements of 320 nm absorption changes induced by a train of saturating laser flashes (FWHM:7 ns) in PS II membrane fragments. It was found: a) Lipophilic quinones complicate the experimental determination of optical changes due the S_i-state transitions because they lead to an additional binary oscillation probably caused by a reductant-induced oxidation of the Fe²⁺ at the PS II acceptor side. b) In principle, a proper separation can be achieved at sufficiently high K₃Fe(CN)₆] concentrations. c) An unequivocal deconvolution into the difference spectra _i+1j of flash train-induced optical changes which are exclusively due to S_i-state transitions is impossible unless the Kok parameters , and S_i]₀ can be determined by an independent method.Measurements of the oxygen yield induced by a flash train reveals, that in thylakoids and PS II membrane fragments S_i is the stable state of dark adapted samples even at alkaline pH (up to pH=9). However, in PS II membrane fragments at pH>7.7 the misses probability markedly increases, in contrast to the properties of intact thylakoids. Based on these data the possibility is discussed that an equilibrium exists of two types of S₂-states with different properties.Abbreviations DCIP 2,6-dichlorophenolindophenol - DPC 1,5-diphenylcarbazide - Q_A.Q_B primary and secondary plastoquinone of PS II - Ph-p-BQ phenyl-p-benzoquinone - PS II photosystem II - S₁ redox state of the catalytic site of water oxidation - Z redox component connecting P680 with the catalytic site of water oxidation Presented at the Japan/US Binational Seminar on Solar Energy Conversion: Photochemical Reaction Centers and Oxygen Evolving Complexes of Plant Photosynthesis, Okazaki, Japan, March 17–21, 1987.