Electron transfer in the heliobacterial reaction center:: evidence against a quinone-type electron acceptor functioning analogous to A1 in photosystem I

Membrane fragments from Heliobacillus mobilis were characterized using time resolved optical spectroscopy and photovoltage measurements in order to detect a possible participation of menaquinone (MQ), functioning analogous to the phylloquinone A(1) in photosystem I, as intermediate in electron transfer from the primary acceptor A(0) to the iron-sulfur cluster F-X in the photosynthetic reaction center. The spectroscopic data obtained exclude that electron transfer from a semiquinone anion MQ(-) to F-X occurred in the time window from 2 ns to 4 mu S, where it would be expected in analogy to photosystem I. In the case of a prereduction of F-X, only the primary pair P798(+)A(0) was formed. The photovoltage data yielded a single kinetic phase with a time constant of 700 ps for the transmembrane electron transfer beyond A(0); the relative amplitude of this phase suggests that it reflects electron transfer from A(0)(-) to FX. (C) 1998 Published by Elsevier Science B.V.