Electric Field Effects on Steady State and Time Resolved Fluorescence from Photosynthetic Reaction Centers

Abstract

A promising approach to identifying the primary electron acceptor and thereby the mechanism of primary charge separation in photosynthetic reaction centers (RCs) is the orientational determination of the dipole moment of the primary radical pair. This can be achieved by studying the angular dependence of the primary charge separation rate in an external electric field as reflected in the Dichroic Excitation spectrum of the electric Field modulated Yield of the prompt fluorescence (DELFY).

For RCs from Rb.sphaeroides R-26 steady state, low temperature DELFY experiments point (within an angle of 5°) to the orientation of the dipole moment of P⁺H⁻ _A. Time resolved fluorescence measurements revealed that the major contribution to the steady state fluorescence quantum yield and to the electric field effect thereon originates from a slow component with a lifetime of ≃ 300 ps at 80K. This fluorescence component is by two orders of magnitude slower than the primary charge separation rate measured in absorption and might originate from a small (≃3%) subset of RCs characterized by slow, unistep charge separation.

Recent transient absorption measurements in electric fields revealed a significant reduction of the quantum yield of P⁺H⁻ _A formation within 30ps demanding a fast loss channel. In order to achieve dynamic competition with fast charge separation in the majority of RCs a more efficient loss channel than internal conversion is required, such as fast parking or trapping of excitation energy. From such a field modulated trapping state with the ability to fluoresce one would expect the observed loss of P⁺H⁻ _A quantum yield accompanied by a slow fluorescence component, which should be stronger than the measured one, however. This model would imply direct charge separation from ¹P* to P⁺H⁻ _A in one step in the majority of RCs. Electric field induced charge separation to the B-branch forming P⁺B⁻ _B would better comply with the data, in case its energy is high enough to produce a quadratic field dependence of the delayed recombination fluorescence.