ENERGY-TRANSFER AND EXCITON COUPLING IN ISOLATED B800-850 COMPLEXES OF THE PHOTOSYNTHETIC PURPLE SULFUR BACTERIUM CHROMATIUM TEPIDUM - THE EFFECT OF STRUCTURAL SYMMETRY ON BACTERIOCHLOROPHYLL EXCITED-STATES

Energy transfer and exciton coupling in isolated B800-850 complexes fr om the purple sulfur bacterium Chromatium tepidum were studied by mean s of spectrally resolved absorbance difference spectroscopy with a tim e resolution of 200 fs. Energy transfer from bacteriochlorophyll (BChl ) 800 to BChl 850 was found to occur with a time constant of 0.8-0.9 p s. Remarkably, the amplitude of the absorbance changes of BChl 850 was 4 times larger than that of BChl 800. By relating this result to the crystal structure of B800-850 complexes of Rhodopseudomonas acidophila (MacDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless , A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs; N. W. Nature 1995, 374, 517), it was concluded that the spectral properties of BChl 850 are m ainly determined by strong exciton interactions between BChl 850 molec ules in a circular symmetric arrangement, which lead to concentration of the oscillator strength in a few optically allowed transitions, cor responding to delocalized eigenstates, In BChl 850, a rapid red shift of the bleaching was observed. This relaxation process may be ascribed either to vibrational relaxation or exciton scattering. A similar red shift appears to occur in BChl 800.