EFFECTS OF APPLIED ELECTRIC-FIELDS ON THE QUANTUM YIELDS FOR THE INITIAL ELECTRON-TRANSFER STEPS IN BACTERIAL PHOTOSYNTHESIS .2. DYNAMIC STARK-EFFECT

The quantum yield of the initial charge separation steps in bacterial photosynthetic reaction centers has been shown to be reduced in an app lied electric field [Part I, Lao et al., J. Phys. Chem. 97 (1993) 1316 5]. The mechanism of this quantum yield failure is examined further by measuring the orientations of the subpopulations which return to the ground state in an electric field. Information on the orientations of these subpopulations can be obtained by measuring the Stark effect spe ctrum of the transient population, the dynamic Stark spectrum, whose l ineshape is sensitive to orientation. This is a generally useful metho d, whose application is developed for general cases. It is shown that considerably more information on orientational subpopulations can be o btained than by conventional photoselection or dichroism methods. In t he case of reaction center quantum yield failure, the dynamic Stark sp ectrum is analyzed to extract information on the absolute orientations of electric dipoles which lead to quantum yield failure. A numerical procedure using the maximum entropy method is developed to map out the most unbiased orientation distribution function from the dynamic Star k spectrum. The distribution of the transient orientational subpopulat ion depends on the magnitude of the interaction between the applied fi eld and the transient dipole moment(s) associated with the electron tr ansfer intermediate(s) responsible for quantum yield failure. The resu lting orientation distribution function suggests that at least two ele ctric-field-dependent mechanisms are important. Based on the X-ray str ucture of the reaction center, the results are analyzed in terms of co ntributions from charge-separated states involving internal charge sep aration within the special pair (P+P-), the monomeric bacteriochloroph yll (P+B-) and bacteriopheophytin (P+H-), each of which can provide fi eld-sensitive shunts to the ground state. Possible relationships with the dynamics of mutants which affect the redox potential of participan ts in electron transfer are discussed.