MODELS FOR BACTERIAL PHOTOSYNTHESIS: ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET BACTERIOPHEOPHYTIN TO METHYL VIOLOGEN AND m‐DINITROBENZENE

Abstract

Abstract. As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC12) and to m‐dinitrobenzene (m‐DNB) in solution. Both MVC12 and m‐DNB cause reductions in the lifetime of the first excited singlet state of BPh (BPh*), in the fluorescence quantum yield, and in the quantum yield of the triplet state, BPh +. The quenching of BPh* probably results from electron transfer, which generates short‐lived radical pairs involving the BPh radical cation (BPh+) and the reduced form of the quencher. Electron transfer from BPh* is thermodynamically favorable, but that from BPhT is not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC12 or m‐DNB. Measurements of the quantum yield of the free BPh+ radical indicate that about 3/4 of the [BPh+ MV+] radical pairs decay by reverse electron transfer, rather than dissociating to give the free radicals. Essentially all of the [BPh+m‐DNB +] radical pairs must decay by reverse electron transfer, because free BPh+ cannot be detected in this case. From these data, we estimate the rate constants for the reverse electron transfer reactions. The higher probability of dissociation in the [BPh+ MV+] radical pair can be explained by coulombic repulsion.The rate of the primary electron transfer reaction in photosynthetic bacteria is comparable to that of forward electron transfer in the BPh* collision complexes. Reverse electron transfer, however, is at least 103‐times slower in the radical pair formed in the bacterial reaction center than it is in [BPh+m‐DNB−], and more than 104‐times slower than in [BPh+ MV+]. The explanation for this dramatic and crucially important difference remains unclear, but several possibilities are discussed.