Electron transfer in reaction center core complexes from the green sulfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum

Electron transfer in reaction center core (RCC) complexes from the green su lfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum was studied by measuring flash-induced absorbance changes. The first preparation conta ined approximately three iron-sulfur centers, indicating that the three put ative electron accepters F-X, F-A, and F-B were present; the Chl. tepidum c omplex contained on the average only one. In the RCC complex of Ptc. aestua rii at 277 K essentially all of the oxidized primary donor (Ps40(+)) create d by a flash was rereduced in several seconds by N-methylphenazonium methos ulfate. In RCC complexes of Chl. tepidum two decay components, one of 0.7 m s and a smaller one of about 2 s, with identical absorbance difference spec tra were observed. The fast component might be due to a back reaction of P8 40(+) with a reduced electron acceptor, in agreement with the notion that t he terminal electron accepters, F-A and F-B, were lost in most of the Chl. tepidum complexes. In both complexes the terminal electron acceptor (F-A or F-B) could be reduced by dithionite, yielding a back reaction of 170 ms wi th P840(+). At 10 K in the RCC complexes of both species P840(+) was reredu ced in 40 ms, presumably by a back reaction with F-X(-). In addition, a 350 mu s component occurred that can be ascribed to decay of the triplet of P8 40, formed in part of the complexes. For P840(+) rereduction a pronounced t emperature dependence was observed, indicating that electron transfer is bl ocked after F-X at temperatures below 200 K.