Effect of neighboring charges and external fields on photosynthetic electron transfer

We investigate the influence of neighboring charges and external electric fields on the rate of an electron-transfer step in bacterial photosynthesis. It is shown that reaction center geometry plays a crucial role in determining the dynamics of photosynthetic electron transfer, by modulating not only the tunneling matrix element, but also, through the coulombic interaction, the Franck-Condon factor. An estimate is made of the intersite energy difference under operative conditions, and the relationship between this quantity and reaction center geometry is discussed. The photosynthetic reduction of the primary reaction center quinone in the presence of reduced secondary quinone is studied. Constraints are placed on the location of the secondary quinone in the reaction center, and the temperature dependence of the transfer rate is predicted. Finally, the effect of externally applied electric fields is studied, and it is shown that this leads to a method for testing the hypothesis that the transfer of an electron from bacteriopheophytin to ubiquinone is an activationless process.