ANTENNA ORGANIZATION IN PURPLE BACTERIA INVESTIGATED BY MEANS OF FLUORESCENCE INDUCTION CURVES

Fluorescence induction curves of purple bacteria (Rs. rubrum, Rps. vir idis and Rb. capsulatus) were measured in the sub-millisecond time ran ge employing a xenon flash technique. The induction curves of all thre e species displayed a sigmoidal shape. Analysis of the curves showed t hat 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-containi ng species Rb. capsulatus. This result demonstrates that LHC 2 (B800-8 50) acts as a sort of insulator between photosynthetic units. Assuming a coordination number of 6 in the LHC 1-containing species the mean s ingle step energy transfer time between adjacent LHC 1 can be estimate d to be 4-5 ps. This is not perfectly compatible with the much faster Forster transfer rate of <1ps that follows from the minimal chromophor e-chromophore distances estimated from digital image processing of mic rographs from stained membranes. It thus may be concluded that the pho tosynthetic 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.