NONEXPONENTIAL FLUORESCENCE DECAY IN REACTION CENTERS OF RHODOBACTER-SPHAEROIDES REFLECTING DISPERSIVE CHARGE SEPARATION UP TO 1 NS

The nonexponential fluorescence decay pattern of the primary donor sta te P-1 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 ''inte rmediate'' fluorescence components (approximate to 40 ps to 1 ns) refl ect (i) the decay of P-1 due to slow charge separation or (ii) the th ermodynamic equilibrium between P-1 and an energetically relaxing P+( H?A-) state (H-A denoting bacteriopheophytin). Such a contribution fro m delayed emission of P+HA- is identified by manipulating the lifetime of this state from approximate to 100 ps (in chinone-containing RC) t o approximate to 15 ns (in chinone-depleted RC). The key observation i s that prompt fluorescence components dominate in the time range up to approximate 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 mino rity of similar to 2% of the RCs extending over a time window up to ap proximate to 1 ns. The distribution of charge-separation rates depends on the thermal accessibility of the radical pair P+HA- and therefore miners energetic differences of P+HA-: (i) In the majority of RCs the state P+HA- is sufficiently low to ensure fast activationless charge s eparation (approximate to 3 ps), while in a minority of RCs high-lying P+HA- states lead to (ii) activated, slow charge separation and to (i ii) superexchange-mediated charge separation to P+HA-, when P+HA- is t hermally no more accessible. At times longer than 600 ps the fluoresce nce components become sensitive to changes of the lifetime of P+HA- in dicating that delayed emission dominates. The time-dependent decrease of the delayed emission reflects an energetic relaxation of P+HA- due to the conformational response of the protein to charge separation.