The primary and secondary electron acceptors in bacterial photosynthesis: I. A chronological account of their identification by EPR

A chronological account is presented of the work that led to the identification of the primary and secondary accepters in photosynthetic reaction centers of Rb. sphaeroides. Both accepters when reduced give rise to broad EPR signals centered at g = 1.82. When the iron is removed from the reaction center, the EPR line, narrowed by approximately two orders of magnitude, is characteristic of a ubisemiquinone. The broad signal in native reaction centers (RCs) is, therefore, ascribed to an unpaired electron being localized on the ubisemiquinone interacting magnetically with the high spin Fe2+. These findings are consistent with the structure of the RC, as determined a decade later by X-ray diffraction.