Energy transfer to the reaction centres in bacterial photosynthesis

Abstract

From a combined study of (1) bacteriochlorophyll fluorescence lifetimes, (2) relative yields and (3) differential absorption changes corresponding to the reaction centres photooxidation, the absolute values of fluorescence lifetimes and quantum yields for two bacteriochlorophyll fractions have been calculated. The main bacteriochlorophyll fraction (∼80–90%) serving as a light-gathering antenna for reaction centresP ₈₉₀ is characterized by dark values of fluorescence lifetimes of the order of 10⁻¹¹ sec and fluorescence yields of 10⁻³. The remaining part of the bulk pigment, not associated withP ₈₉₀ as far as excitation energy transfer is concerned, has an approximately constant fluorescence yield of about 5–8% and lifetime of about 10⁻⁹ sec. Basing on these results, excitation jump times and intermolecular coupling energies were estimated to be 10⁻¹³ sec and 10⁻² ev respectively. The conclusion is made that excitation energy transfer in the main part of bacteriochlorophyll occurs by the exciton mechanism at moderate intermolecular energies.