DISTRIBUTION OF ENERGY ABSORBED BY PURPLE BACTERIA BETWEEN VARIOUS PATHS OF DEEXCITATION

The existence of pools of bacteriochlorophyll molecules, characterized by various absorption and emission maxima and different orientations of their transition moments with respect to the membrane plane, was shown. The polarized spectra suggest that in addition to the light-harvesting LH1 B875 complexes, other antenna complexes (B806 and complexes absorbing in the long-wavelength region) are present in Rhodospirillum rubrum. The ratio between the yield of fluorescence polarized parallel and perpendicular to the direction of the electric vector of the exciting light (376 nm) and the film axis is about 1.8 for bacteriochlorophyll emission at 892 nm and about 0.14 for that at 928 nm. In the parallel component, 28% of the energy absorbed in the near-IR bands of bacteriochlorophyll is converted into heat and 72% is used for fluorescence emission and photochemistry, whereas in the perpendicular component, the corresponding values are about 60% and 40% respectively. We conclude that the mutually parallel bacteriochlorophyll antenna chromophores lose less energy by thermal dissipation than chromophores with a lower degree of orientation. This suggests a very efficient trapping in the reaction centres of that part of the excitation energy which migrates between chromophores having mutually parallel transition moments.