Organization of the antenna bacteriochlorophylls around the reaction center of Rhodopseudomonas viridis investigated by photoselection techniques

Abstract

The orientation of the long-wavelength (Q y) transition dipoles of the antenna bacteriochlorophylls was examined in isolated quantasomes of Rhodopseudomonas viridis. A quantasome consists of a reaction center surrounded by six antenna complexes, each of which probably contains four molecules of bacteriochlorophyll b. We measured the photodichroism of the absorbance changes associated with oxidation of the primary electron-donor in the reaction center (P-960), when the long-wavelength absorption band of the antenna was excited selectively with polarized light. The polarization of the fluorescence from the antenna was also measured. Similar measurements were made with isolated reaction centers, in which P-960 could be excited directly, and with intact chromatophores. Reaction centers gave high polarization values for the fluorescence and the photodichroism; chromatophores and quantasomes both gave low values. The results are consistent with the view that the long-wavelength transition dipoles of the antenna molecules lie in a common plane with the corresponding dipole of P-960, but have no preferred orientation in this plane. The antenna thus appears to be circularly degenerated about P-960.