DIELECTRIC ASYMMETRY IN THE PHOTOSYNTHETIC REACTION-CENTER

Although the three-dimensional structure of the bacterial photosynthet ic reaction center (RC) reveals a high level of structural symmetry, w ith two nearly equivalent potential electron transfer pathways, the RC is functionally asymmetric: Electron transfer occurs along only one o f the two possible pathways. In order to determine the origins of this symmetry breaking, the internal electric field present in the RC when charge is separated onto structurally characterized sites was probed by using absorption band shifts of the chromophores within the RC. The sensitivity of each probe chromophore to an electric field was calibr ated by measuring the Stark effect spectrum, the change in absorption due to an externally applied electric field. A quantitative comparison of the observed absorption band shifts and those predicted from vacuu m electrostatics gives information on the effective dielectric constan t of the protein complex. These results reveal a significant asymmetry in the effective dielectric strength of the protein complex along the two potential electron transfer pathways, with a substantially higher dielectric strength along the functional pathway. This dielectric asy mmetry could be a dominant factor in determining the functional asymme try of electron transfer in the RC.