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

Abstract

The possibility to determine the difference spectra Δεi+1jλ of each univalent redox step Si→Si+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 Si-state transitions because they lead to an additional binary oscillation probably caused by a reductant-induced oxidation of the Fe(2+) at the PS II acceptor side. b) In principle, a proper separation can be achieved at sufficiently high K3[Fe(CN)6] concentrations. c) An unequivocal deconvolution into the difference spectra Δεi+1jλ of flash train-induced optical changes which are exclusively due to Si-state transitions is impossible unless the Kok parameters α, β and [Si]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 Si 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 α 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.