PICOSECOND KINETICS OF LIGHT EXCITATIONS IN PHOTOSYNTHETIC PURPLE BACTERIA IN THE TEMPERATURE-RANGE OF 300-4 K

Comparative measurements of time- and spectrally resolved picosecond fluorescence of the photosynthetic purple bacterium Rhodospirillum rubrum in a wide temperature range from room temperature down to 4 K have been performed with different excitation wavelengths within the main infrared absorption band employed. Several kinetically different spectral components have been characterized in the fluorescence decay at low temperatures. This gives substantial support to the viewpoint that the B880 light-harvesting antenna band of R. rubrum (and probably the core antenna of other purple bacteria) consists of more than just two spectral forms, as has been suggested by several authors. The rates of the antenna singlet excitation quenching by reaction centres in different states have been determined. Although almost unchanged in the temperature range from 300 to 77 K, these rates decrease by 2 to 4 times on temperature lowering down to 4 K. It was found that at 4 K, the oxidized primary donor is a stronger fluorescence quencher than its triplet state.