Experimental Evidence of Oligomeric Organization of Antenna Bacteriochlorophyll C in Green Bacterium Chloroflexus Aurantiacus by Spectral Hole Burning

Spectral hole burning has been used to prove experimentally the existence in natural antenna of one of the predicted structural optimizing factors--antenna pigment oligomerization [J. Theor. Biol. 140 (1989) 167]--ensuring high efficiency of excitation energy transfer from antenna to reaction center. This point has been examined for the chlorosomal antenna of green bacterium Chloroflexus aurantiacus by hole burning in fluorescence excitation and emission spectra of intact cells at 1.8 K. The persistent hole spectra have been found to be consistent with a strongly exciton-coupled bacteriochlorophyll c (BChl c) chromophore system. The lowest exciton state of BChl c oligomers has been directly detected and separated as the lowest energy inhomogeneously broadened band (FWHM approximately 90 cm-1, position of maximum, at approximately 752 nm) from the near-infrared BChl c band (FWHM approximately 350 cm-1, position of maximum, at approximately 742 nm) of 1.8 K excitation spectrum.