Electron-paramagnetic-resonance measurements of the electron-transfer components of the reaction centre of Rhodopseudomonas viridis. Oxidation–reduction potentials and interactions of the electron acceptors

Abstract

Oxidation–reduction potentiometry was carried out on Rhodopseudomonas viridis chromatophores. Measurements of e.p.r. signals of the semiquinone–iron type at g=1.82 have revealed a more complex situation than previously reported. The presence of three different components is indicated. The midpoint potential (E_m) of the primary acceptor quinone/semiquinone couple was found to be approx. −165mV at pH10, with a pK being reached at around pH7.5. The primary acceptor also accepts a second electron with an E_m of −525mV, but this redox transition exhibits a hysteresis effect. Interaction effects indicate the presence of another component with E_m values at pH10 of approx. −165mV (pK reached at around pH7.5) for single reduction and −350mV (pK at pH10 or greater) for double reduction. It is suggested that this component is the secondary acceptor. Another semiquinone–iron-type component which gives a g=1.82 signal is also present. This component is distinguishable from the primary acceptor by its e.p.r. spectrum, which shows a double peak at g=1.82 and a g_x line at g=1.76. This component has E_m values at pH10 for single and double reduction of −15mV and approx. −150mV respectively. Both of these E_m values are pH-dependent. The presence of an interaction between this component and the photoreduced primary acceptor indicates the close proximity of these components. However, the midpoint potential of this component indicates a function as a secondary electron-transport component rather than an electron acceptor in the reaction centre. The dependence of the bacteriopheophytin intermediate (I) doublet e.p.r. signal on the presence of the semiquinone–iron form of the primary acceptor is demonstrated. The midpoint potential of the I/I⁻ couple is estimated to be lower than −600mV.