Macroscopic and Microscopic Estimates of the Energetics of Charge Separation in Bacterial Reaction Centers

Two approaches for calculating the free energies of transient radical-pair states in bacterial reaction centers are discussed. Although macroscopic models that assign a homogenous dielectric constant to the protein and solvent are major oversimplifications, they help to clarify the importance of considering the self-energies of the charged species, and to put limits on the energetics of the charge-separation processes. The microscopic Protein-Dipoles-Langevin-Dipoles (PDLD) approach provides a much more realistic treatment of dielectric effects, but requires lengthy calculations that depend on numerous interrelated factors. Calculations by both approaches indicate that, in Rhodopseudomonas viridis reaction centers, the state P+B- generated by movement of an electron from the primary electron donor (P) to a neighboring bacteriochlorophyll (B) lies close to the excited state P* in energy, where it possibly could serve as an intermediate in electron transfer to the bacteriopheophytin (H). This conclusion agrees with previous free-energy-perturbation calculations and indicates that any model (macroscopic or microscopic) that includes all the relevant contributions and reproduces the energy of the relaxed P+H- should find the relaxed P+B- state near P*. In addition, the macroscopic model shows that electron transfer from P* to H is likely to be exothermic even in the absence of a strong field from the atomic charges of the protein.