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

The singlet-triplet splitting J of the radical pair P+H- in the photos ynthetic reaction center critically depends on the energy separation b etween 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 theoretic al approach, such a shift should enlarge J to values which do not allo w singlet-triplet-mixing in P+H-, thereby inhibiting the triplet recom bination 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 th e energy of P+H- is predicted in an extended theoretical treatment of J taking into account nuclear oscillations. Following the extended tre atment, J is much less sensitive to the energy of P+H-. This has impor tant consequences for the discussion of J in the context of primary ch arge separation and also for the values of J to be expected in reactio n centers in which the energetics have been altered by chemical or gen etic modifications.