Photochemical Electron Transport in Photosynthetic Reaction Centers: IV. Observations Related to the Reduced Photoproducts

Abstract

The formation and dissipation of reduced photoproducts in photochemical reaction centers from Rhodopseudomonas spheroides has been studied in three independent ways: by direct chemical reduction, by photochemical reduction (illuminating reaction centers in the presence of weak reductants), and by adding electron acceptors to illuminated reaction centers to reverse the reduction. In every case the reduction is attended by the appearance of an absorption band at 450 nm and the bathochromic shift of a band centered at 305 nm. Both reduction and oxidation of reaction centers, and also photochemical oxidoreduction, cause bathochromic shifts of absorption bands identified with bacteriopheophytin (BPh), and hypsochromic shifts of bands of bacteriochlorophyll (BChl) (P-800 and, in the case of reduction, P-870). Reduction causes relatively large shifts of BPh and small shifts of BChl; the reverse is seen with oxidation and oxidoreduction. Addition of sodium dodecyl sulfate (SDS) to reaction centers suppresses the 450 nm absorption change but not the band shifts associated with BPh and BChl. Under some conditions the 450 nm change and the band shifts show different kinetics, with the kinetics of the band shifts matching those of a transient change in the yield of P-870 fluorescence. New data, on the efficiency of photo-bleaching of P-870 in reaction centers in which part of the P-870 has already been oxidized with ferricyanide, militate against the idea that part of the photochemical bleaching of P-870 is due to reduction of that pigment.