ELECTRIC-FIELD EFFECTS ON STEADY-STATE AND TIME-RESOLVED FLUORESCENCEFROM PHOTOSYNTHETIC REACTION CENTERS

ABSTRACT A promising approach to identifying the primary electron acce ptor and thereby the mechanism of primary charge separation in photosy nthetic reaction centers (RCs) is the orientational determination of t he 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 fluoresce nce (DELFY). For RCs from Rb.sphaeroides R-26 steady state, low temper ature DELFY experiments point (within an angle of 5-degrees) to the or ientation 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 or iginates from a slow component with a lifetime of congruent-to 300 ps at 80K. This fluorescence component is by two orders of magnitude slow er than the primary charge separation rate measured in absorption and might originate from a small (congruent-to 3%) subset of RCs character ized by slow, unistep charge separation. Recent transient absorption m easurements 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 sepa ration in the majority of RCs a more efficient loss channel than inter nal conversion is required, such as fast parking or trapping of excita tion energy. From such a field modulated trapping state with the abili ty to fluoresce one would expect the observed loss of P+H(A)- quantum yield accompanied by a slow fluorescence component, which should be st ronger than the measured one, however. This model would imply direct c harge separation from 1P to P+H(A)- in one step in the majority of RC s. Electric field induced charge separation to the B-branch forming PB(B)- would better comply with the data, in case its energy is high en ough to produce a quadratic field dependence of the delayed recombinat ion fluorescence.