Electrostatic calculations of the pKa of ionizable groups in the reaction center of Rhodopseudomonas (Rps.) viridis were carried out to investigate three possible mechanisms for proton transfer to the singly reduced acceptor ubiquinone QB. The program DelPhi, which solves the Poisson–Boltzmann equation given the distribution of charges and dielectric boundaries, was used to determine the electrostatic potential. The shift in pKa of the titratable residues in the QB binding pocket in response to the one-electron reduction and following protonation of QB was obtained from calculated interactions with the reaction field, background protein dipoles, charged cofactors, and other ionizable residues. A limited number of bound waters was also included in the computations as titrating sites. Their titration behavior was shown to be strongly coupled to neighboring ionizable sites. The results show that strong electrostatic interaction between the radical anion QB−· and a neighboring serine residue (SER L 223) as well as the protein environment stabilize a system in which the incoming proton is localized on serine and only shared in a hydrogen-bonding relationship with QB−·. These results hint to the possibility that actual proton transfer to QB−· only occurs after a second negative charge has been added to the system through transfer of a second electron either to the menaquinone QA, with formation of the QB−· QA−· system, or to QB−·, leading to the doubly reduced QB2−. This preposition is consistent with spectroscopical and electron nuclear double resonance (ENDOR) experimental results for bacterial reaction centers (RCs) failing to find evidence for the existence of the protonated semiquinone QBH·. © 1993 John Wiley & Sons, Inc.