Spectroscopy on single light-harvesting complexes at low temperature

The fluorescence of individual light-harvesting 2 complexes from Rhodopseud omonas acidophila has been observed by confocal microscopy in a temperature range between 300 and 7 K. Under ambient conditions, changes in the polari zation of the fluorescence emission of single complexes on a time scale fro m milliseconds to seconds are found. In the temperature range between 250 a nd 100 K most complexes emit fluorescence with a temporally stable linear f luorescence polarization. At temperatures below 70 K, spectral diffusion is found to dominate the dynamics of the fluorescence intensity and polarizat ion. The increase in photostability of single complexes at low temperature allows the detection of fluorescence emission spectra of single complexes. A marked variation in the shape and the position of the spectra is found. T he results are discussed by considering static and dynamic disorder within the B850 aggregate of the light-harvesting complexes.