Delayed fluorescence from the photosynthetic reaction center measured by electronic gating of the photomultiplier

The decay of the delayed fluorescence (920 nm) of reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides R26 in the P+QA- charge-separated state (P and QA are the primary donor and quinone, respectively) has been monitored in a wide (100 ns to 100 ms) time range. The photomultiplier (Hamamatsu R3310-03) was protected from the intense prompt fluorescence by application of gating potential pulses (-280 V) to the first, third, and fifth dynodes during the laser pulse. The gain of the photomultiplier dropped transiently by a factor of 1 × 10^6. The delayed fluorescence showed a smooth but nonexponential decay from 100 ns to 1 ms that was explained by the relaxation of the average free energy between P* and P+QA- changing from -580 to -910 meV. This relaxation is due to the slow protein response to charge separation and can be described by a Kohlrausch relaxation function with time constant of 65 (mu)s and a stretching exponent of (alpha)= 0.45.