Fluorescence of bacteriochlorophyll as related to the photochemistry of chromatophores of photosynthetic bacteria

Time courses and the emission spectra of fluorescence and light-induced absorption changes of P890 in chromatophores of the photosynthetic bacteria Chromatium D, Rhodopseudomonas spheroides and Rhodospirillum rubrum were investigated.

The time course of fluorescence in chromatophores was separated into two phases, i.e. an initial rapid rise (?_i) and a subsequent slow increase towards a steady level of emission (?_v). The ?_i and the ?_v components showed different emission spectra having different peak position. The ?_v component was emitted from the longest wavelength-absorbing form of bulk bacteriochlorophyll (B890), the ?_i component from both B890 and B850.

The magnitude of the ?_v component depended on experimental conditions controlling the states of the cyclic electron transport in chromatophores, including changes in levels of redox potential of the medium, additions of electron donors and inhibitors. The magnitude of the ?_i component was not affected by these experimental conditions. It was, therefore, concluded that only the ?_v component is related to the cyclic electron transport, and that the magnitude of ?_v is controlled by the oxidation-reduction state of the primary electron acceptor for the photochemical reaction center in chromatophores.