Effects of applied electric fields on the quantum yields for the initial electron transfer steps in bacterial photosynthesis. II. 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 applied electric field [Part I, Lao et al., J. Phys. Chem. 97 (1993) 13165]. 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 spectrum of the transient population, the dynamic Stark spectrum, whose lineshape is sensitive to orientation. This is a generally useful method, whose application is developed for general cases. It is shown that considerably more information on orientational subpopulations can be obtained than by conventional photoselection or dichroism methods. In the case of reaction center quantum yield failure, the dynamic Stark spectrum 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 Stark spectrum. The distribution of the transient orientational subpopulation depends on the magnitude of the interaction between the applied field and the transient dipole moment(s) associated with the electron transfer intermediate(s) responsible for quantum yield failure. The resulting orientation distribution function suggests that at least two electric-field-dependent mechanisms are important. Based on the X-ray structure of the reaction center, the results are analyzed in terms of contributions from charge-separated states involving internal charge separation within the special pair (P ÷P-), the monomeric bacteriochlorophyll (P+B-) and bacteriopheophytin (P+H-), each of which can provide field-sensitive shunts to the ground state. Possible relationships with the dynamics of mutants which affect the redox potential of participants in electron transfer are discussed.