WHAT CAN BE LEARNED FROM THE SINGLET-TRIPLET SPLITTING OF THE RADICAL PAIR P+H- IN THE PHOTOSYNTHETIC REACTION-CENTER - CONCLUSIONS FROM ELECTRIC-FIELD EFFECTS ON THE P+H- RECOMBINATION DYNAMICS

The singlet-triplet splitting J of the radical pair P+H- in the photosynthetic reaction center critically depends on the energy separation between P+H- and the triplet state P-3*. Due to its large dipole moment, the energy of P+H- can be considerably shifted by the application of an external electric field. Following a simple perturbation theoretical approach, such a shift should enlarge J to values which do not allow singlet-triplet-mixing in P+H-, thereby inhibiting the triplet recombination channel P+H- --> P-3*. In contrast to this prediction, there combination dynamics of P+H- and the yield of P-3* in reaction centers of Rb. sphaeroides were found to change only slightly in an external electric field. This insensitivity of the recombination dynamics to the energy of P+H- is predicted in an extended theoretical treatment of J taking into account nuclear oscillations. Following the extended treatment, J is much less sensitive to the energy of P+H-. This has important consequences for the discussion of J in the context of primary charge separation and also for the values of J to be expected in reaction centers in which the energetics have been altered by chemical or genetic modifications.