Electric-field dependence of the quantum yield in reaction centers of photosynthetic bacteria

Multilayer Langmuir-Blodgett films of reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides have been fabricated with partial net orientation. The films showed substantial electrical response under pulsed illumination. From measurements of the light-induced voltage generated across the Langmuir-Blodgett film, we have succeeded in quantitating the electric-field dependence of the quantum yield of charge separation in photosynthesis. The results presented here are compared with our previous determination of the field effect on quantum yield, in which flash-activated charge separation as a function of the applied field was assayed by the extent of bacteriochlorophyll dimer, (BChl)2, oxidation measured optically at 860 nm. The two methods provided consistent dependencies of quantum yield on applied electric field. Analysis of the data reveals that the quantum yield of (BChl)⨥2BPhQ⨪A formation decreases from a value of 0.96 at zero applied field to about 0.75 for a field of 120 mV/nm vectorially directed to hinder light-activated electron transfer. For oppositely applied fields, the quantum yield saturates at unity. The source of the effects is considered to reside in the electric field dependence of the free-energy difference between the energy levels that are involved in the initial charge separation between the (BChl)2 in the first singlet excited state, (BChl)∗2, through the bacteriopheophytin, BPh, to the primary ubiquinone, QA. Possible contributions to the field-induced loss of quantum yield of (BChl)⨥2BPhQ⨪A formation are: (1) a decrease in the free-energy gap between the states (BChl)∗2 and (BChl)⨥2BPh⨪QA, leading to an increased rate of decay via the excited singlet state back to the ground state; (2) a stimulated return from (BChl)⨥2BPh⨪QA directly or via the (BChl)2 triplet state to the ground state and (3) an impeded electron transfer from (BChl)⨥2BPh⨪QA to (BChl)⨥2BPhQ⨪A. These possibilities are discussed. Correlation of the electrical response with measurements of the photo-induced absorbance change allows determination of the projection of the electron-transfer distance on the normal to the plane of the film, which is in good agreement with previous measurements using different techniques.