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 A₁ in photosystem I, as intermediate in electron transfer from the primary acceptor A₀ 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 μs, where it would be expected in analogy to photosystem I. In the case of a prereduction of F(X), only the primary pair P798⁺A₀ ^- was formed. The photovoltage data yielded a single kinetic phase with a time constant of 700 ps for the transmembrane electron transfer beyond A₀; the relative amplitude of this phase suggests that it reflects electron transfer from A₀ to F(X).