FLUORESCENCE AND PHOTOBLEACHING DYNAMICS OF SINGLE LIGHT-HARVESTING COMPLEXES

Single light-harvesting complexes LH-2 from Rhodopseudomonas acidophil a were immobilized on various charged surfaces under physiological con ditions, Polarized light experiments showed that the complexes were si tuated on the surface as nearly upright cylinders, Their fluorescence lifetimes and photobleaching properties were obtained by using a confo cal fluorescence microscope with picosecond time resolution, initially all molecules fluoresced with a lifetime of I +/- 0.2 ns, similar to the bulk value, The photobleaching of one bacteriochlorophyll molecule from the 18-member assembly caused the fluorescence to switch Off com pletely, because of trapping of the mobile excitations by energy trans fer, This process was linear in light intensity, On continued irradiat ion the fluorescence often reappeared, but all molecules did not show the same behavior, Some LH-2 complexes displayed a variation of their quantum yields that was attributed to photoinduced confinement of the excited states and thereby a diminution of the superradiance. Others s howed much shorter lifetimes caused by excitation energy traps that ar e only approximate to 3% efficient. On repeated excitation some molecu les entered a noisy state where the fluorescence switched on and off w ith a correlation time of approximate to 0.1 s. About 490 molecules we re examined.