Photosynthetic Reaction Center: A Nonergodic, Dynamically Anisotropic, and Nonlinear Charge-Transport Engine

We ask the question of what is special about photosynthetic reaction center proteins as media for transporting electrons across the lipid membrane. Ergodicity is broken down for the statistics of electrostatic fluctuations in the heliobacterial reaction center studied here by atomistic simulations. It is not restored on the simulation time scale of ∼1 μs, and it allows low activation barriers for electron hops. The medium dynamics are highly anisotropic (depending on the oxidation state) at cofactor sites, allowing a unidirectional flow of charge. This nonlinear protein response to altering the oxidation state combines with the coupling of cofactor polarizabilities to strong intraprotein electric fields to produce nonparabolic and nonergodic free-energy surfaces of electron transfer, allowing low-barrier charge conductivity in proteins.