Photochemical electron transport in photosynthetic reaction centers from Rhodopseudomonas spheroides. II. Interaction with external electron donors and acceptors and a reevaluation of some spectroscopic data.

Photochemical reaction centers prepared from Rhodopseudomonas spheroides were treated with reduced cytochrome c (cyt c), and in some cases with ubiquinone (UQ), and illuminated. The light-induced oxidation of cy and reduction of UQ were observed, and also the variations in fluorescence of P870. These observations indicated that each reaction center contains a primary photochemical electron acceptor capable of holding just one electron. Depending on the method of preparation, the reaction centers may also contain secondary electron acceptor pools consisting mainly of UQ. The role of native UQ as an electron acceptor could be duplicated by added UQ. The yield of P870 fluorescence increased by a factor of 3-4, at most, during illumination of reaction centers in the presence of an electron donor such as reduced cyt. This suggests that the quantum efficiency for the primary photoact is about 0.7, rather than 0.9-1.0 as concluded in the past from optical absorption measurements. The apparent quantum efficiency for the oxidation of cyt by illuminated reaction centers can be increased by the addition of UQ and is decreased at higher concentrations of the detergent lauryl dimethylamine oxide (LDAO). These treatments do not affect the quantum efficiency of P870 oxidation, measured in the absence of cyt.