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MODELS FOR BACTERIAL PHOTOSYNTHESIS: ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET BACTERIOPHEOPHYTIN TO METHYL VIOLOGEN AND m ‐DINITROBENZENE
Author(s) -
Holten Dewey,
Windsor Maurice W.,
Parson William W.,
Gouterman Martin
Publication year - 1978
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1978.tb07734.x
Subject(s) - chemistry , electron transfer , photochemistry , quantum yield , singlet state , radical , quenching (fluorescence) , reaction rate constant , excited state , yield (engineering) , photoinduced electron transfer , fluorescence , atomic physics , kinetics , organic chemistry , thermodynamics , physics , quantum mechanics
. As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC1 2 ) and to m ‐dinitrobenzene ( m ‐DNB) in solution. Both MVC1 2 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 BPh T is not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC1 2 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 10 3 ‐times slower in the radical pair formed in the bacterial reaction center than it is in [BPh + m ‐DNB − ], and more than 10 4 ‐times slower than in [BPh + MV + ]. The explanation for this dramatic and crucially important difference remains unclear, but several possibilities are discussed.