The role of the protein in the stabilization of the charge separated states in photosynthetic reaction centers
Electron transfer from the excited singlet state of a bacteriochlorophyll dimer (P) to a bacteriopheophytin (HL) 18 Å away occurs within 2 picoseconds even at cryogenic temperatures in photosynthetic reaction centers. Estimates of the reaction field energy that would stabilize the ion pair P+HL -, shows that a medium with a dielectric constant of 2 would be insufficient to place the energy of the ion pair below that of the excited singlet state of P. The contribution of additional dielectric screening appears to be small. Thus, the protein must play an active role in stabilizing the charge separated state. A static field due to pre-existing charges and dipoles is calculated to exist within reaction centers which is negative near P and positive near H. This field stabilizes P+HL - so that is lies below P*.
Keywordselectron transfer electrostatics reaction centers
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