ELECTRON-TRANSFER IN THE HELIOBACTERIAL REACTION-CENTER - EVIDENCE AGAINST A QUINONE-TYPE ELECTRON-ACCEPTOR FUNCTIONING ANALOGOUS TO A(1) IN PHOTOSYSTEM-I

Membrane fragments from Heliobacillus mobilis were characterized using time resolved optical spectroscopy and photovoltage measurements in o rder to detect a possible participation of menaquinone (MQ), functioni ng analogous to the phylloquinone A(1) in photosystem I, as intermedia te in electron transfer from the primary acceptor A(0) to the iron-sul fur cluster F-X in the photosynthetic reaction center. The spectroscop ic data obtained exclude that electron transfer from a semiquinone ani on 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 pr ereduction of F-X, only the primary pair P798(+)A(0) was formed. The p hotovoltage data yielded a single kinetic phase with a time constant o f 700 ps for the transmembrane electron transfer beyond A(0); the rela tive amplitude of this phase suggests that it reflects electron transf er from A(0)(-) to FX. (C) 1998 Published by Elsevier Science B.V.