Antenna organization in purple bacteria investigated by means of fluorescence induction curves.

Fluorescence induction curves of purple bacteria (Rs. rubrum, Rps. viridis and Rb. capsulatus) were measured in the sub-millisecond time range employing a xenon flash technique. The induction curves of all three species displayed a sigmoidal shape. Analysis of the curves showed that none of the species examined had an antenna organization of a lake (i.e. unrestricted energy transfer between photosynthetic units). The apparent time constants of inter-unit exciton transfer were estimated to be approximately 24 ps in the case of LHC 1-containing species (Rs. rubrum and Rps. viridis) and 40 ps in the case of the LHC 2-containing species Rb. capsulatus. This result demonstrates that LHC 2 (B800-850) acts as a sort of insulator between photosynthetic units. Assuming a coordination number of 6 in the LHC 1-containing species the mean single step energy transfer time between adjacent LHC 1 can be estimated to be 4-5 ps. This is not perfectly compatible with the much faster Förster transfer rate of <1ps that follows from the minimal chromophore-chromophore distances estimated from digital image processing of micrographs from stained membranes. It thus may be concluded that the photosynthetic units (reaction center plus LHC 1) are loosely arranged in the photosynthetic membrane, like in the fluid-mosaic-membrane model, rather than in a hexagonally crystalline configuration.