Selective photochemical reduction of either of the two bacteriopheophytins in reaction centers of Rps. sphaeroides R‐26

The bacterial photosynthetic reaction center (RC) contains two bacteriopheophytin (BPh) molecules, only one of which (characterized by Q x,y maxima at 540 and 760 nm) has been found to function in the normal photochemistry. We present here the formation of a new RC redox state, in which the apparently inactive BPh (Q x,y maxima at 530 and 752 nm) is selectively trapped in a reduced state by secondary photochemistry. This BPh is found to be associated to a bacteriochlorophyll (BChI), forming a BChl‐BPh complex analogous to the photochemically active BChl‐BPh acceptor complex. Electron transfer between the two BPhs is found not to occur.