Delayed fluorescence from Rhodopseudomonas sphaeroides following single flashes

Abstract

Delayed fluorescence from Rhodopseudomonas sphaeroides chromatophores was studied with the use of short flashes for excitation. Although the delayed fluorescence probably arises from a back-reaction between the oxidized reaction center bacteriochlorophyll complex (P +) and the reduced electron acceptor (X −), the decay of delayed fluorescence after a flash is much faster ( τ 1 2 ≈ 120 μ s ) than the decay of P +X −. The rapid decay of delayed fluorescence is not due to the uptake of a proton from the solution, nor to a change in membrane potential. It correlates with small optical absorbance changes at 450 and 770 nm which could reflect a change in the state of X −. The intensity of the delayed fluorescence is 11–18-fold greater if the excitation flashes are spaced 2 s apart than it is if they are 30 s apart. The enhancement of delayed fluorescence at high flash repetition rates occurs only at redox potentials which are low enough (< + 240 mV) so that electron donors are available to reduce P +X − to PX − in part of the reaction center population. The enhancement decays between flashes as PX − is reoxidized to PX, as measured by the recovery of photochemical activity. Evidently, the reduction of P +X − to PX − leads to the storage of free energy that can be used on a subsequent flash to promote delayed fluorescence. The reduction of P +X − also is associated with a carotenoid spectral shift which decays as PX − is reoxidized to PX. Although this suggests that the free energy which supports the delayed fluorescence might be stored as a membrane potential, the ionophore gramicidin D only partially inhibits the enhancement of delayed fluorescence. With widely separated flashes, gramicidin has no effect on delayed fluorescence. At redox potentials low enough to keep X fully reduced, delayed fluorescence of the type described above does not occur, but one can detect weak luminescence which probably is due to phosphorescence of a protoporphyrin.