Delayed Fluorescence and the Reversal of Primary Photochemistry in Rhodopseudomonas Viridis

Delayed fluorescence from chromatophores of the photosynthetic bacterium Rhodopseudomonas viridis was measured at temperatures below 0°C. A component with a decay half‐time of about 7 ms was found. Its intensity was directly proportional to the number of reaction centers in the P985+·A‐ state. During prolonged illumination it faded as electrons moved forward along the electron transport chain from the primary acceptor, A, (P985+·A‐→P985+·A), and its decay in the dark paralleled the disappearence of the P985+ electron paramagnetic resonance absorption. The data suggest that this component of delayed fluorescence results from a direct reversal of the primary light reaction. While the rate of the P985+middot;A‐→P985·A reaction was almost independent of temperature, delayed fluorescence intensity displayed an apparent activation energy of 0°2 eV. It is concluded that the P985+·A‐→P985·A reaction proceeds by parallel radiative and nonradiative routes. The direct proportionality between delayed fluorescence and the concentration of P985+·A‐ pairs seems to preclude an involvement of triplet‐triplet annihilation or dependence of delayed fluorescence upon the variable prompt fluorescence yield.