Primary event electron transfer model

The extremely rapid rate of formation of bathorhodopsin as compared to isomerization rates observed in model protonated Schiff bases (a factor of 103) suggested the idea that electron transfer between an amino acid residue (e.g., tyrosine or tryptophan) in the protein and the chromophore may catalyse isomerization. Thus, a photoinduced electron transfer leading to a radical anion chromophore, instead of complete cis-trans isomerization, was considered as a plausible alternate mechanism for the primary event [200], This mechanism, however, is difficult to reconcile with the known photoreversibility but thermal irreversibility of the bleaching process. Thermal irreversibility of the light-induced electron transfer would require geometrical separation of donor and acceptor moieties which would then not allow photoreversibility [201]. 

An approach, similar to that of the chelated heme model systems, has been adopted by many researchers to prepare porphyrins with appended quinone groups. Such systems have stimulated interest as possible models for the primary electron transfer event of photosynthesis, where photoinduced charge transfer occurs from excited singlet state chlorophyll donors to nearby quinone acceptors. 

Modeling the Primary Electron Transfer Events of Photosynthesis Michael R. Wasielewski... 

The model was studied varying rate constants and the amount and reduction state of PQ. As the events related to the transfer of excitation are considered to be in equilibrium, the first rate-determining constants are k3 and k4 describing the movement of electron from the primary donor Y to P680 and from Phe" to Q. On the basis of the fluorescence induction curves it was impossible to distinguish which of the two was the dominant resistance. Using different combinations of k3 and k4 and keeping l/k3 + l/k4=const, identical fluorescence transients were calculated. 

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