Nonexponential Fluorescence Decay in Reaction Centers of Rhodobacter sphaeroides Reflecting Dispersive Charge Separation up to 1 ns

摘要

The nonexponetial fluorescence decay pattern of the primary donor state ~1P~* in the reaction center (RC) of Rhodobacter sphaeroides R26 has been investigated in order to identify the origin of such dispersive kinetics. Of particular interest was the open question, whether "intermediate" fluorescence components (≈40 ps to 1 ns) reflect (i) the decay of ~1P~* due to slow charge separation or (ii) the thermodynamic equilibrium between ~1P~* and an energetically relaxing P~+H_A~- state (H_A denoting bacteriopheophytin). Such a contribution from delayed emission of P~+H_A~- is identified by manipulating the lifetime of this state from ≈100 ps (in chinonecontaining RC) to ≈15 ns (in chinone-depleted RC). The key observation is that prompt fluorescence components dominate in the time range up to ≈600 ps at 290 K since they are not affected by the P~+H_A~- lifetime. These components reflect slow charge separation of a minority of ～2% of the RCs extending over a time window up to ≈1 ns. The distribution of charge-separation rates depends on the thermal accessibility of the radical pair P~+B_A~- and therefore mirrors energetic differences of P~+B_A~-: (i) In the majority of RCs the state P~+B_A~- is sufficiently low to ensure fast activationless charge separation (≈3 ps), while in a minority of RCs high-lying P~+B_A~- states lead to (ii) activated, slow charge separation and to (iii) superexchange-mediated charge separation to P~+H_A~-, when P~+B_A~- is thermally no more accessible. At times longer than 600 ps the fluorescence components become sensitive to changes of the lifetime of P~+H_A~- indicating that delayed emission dominates. The time-dependent decrease of the delayed emission reflects an energetic relaxation of P~+H_A~- due to the conformational response of the protein to charge separation.