Reversible conversion of aggregated bacteriochlorophyll c to the monomeric form by 1-hexanol in chlorosomes from Chlorobium and Chloroflexus

Abstract

When isolated chlorosomes from Chlorobium limicola or Chloroflexus aurantiacus are suspended in a solution saturated with 1-hexanol, the far-red absorption band of bacteriochlorophyll c at 750 or 740 nm is converted completely to a band peaking at 670 nm. The cooperation of 9 to 15 hexanol molecules is required to effect this change. This conversion corresponds to a change of the pigment molecules from the aggregated form to the monomeric form in vitro and suggests that hexanol destroys the strong interaction between the chlorosome pigments by the ligation of the hydroxyl oxygen of hexanol to the magnesium atom of the chlorophyll. However, fluorescence from the monomer in the treated chlorosomes is very small in comparison to that from monomer in organic solvent or detergent treated chlorosomes and efficient energy transfer from bacteriochlorophyll c to bacteriochlorophyll a in the hexanol-treated chlorosomes is still observed. When the treated chlorosomes are diluted slowly with buffer by a factor of two or more, the hexanol effect is reversed completely. These results suggest that the red-shifted far-red bands of bacteriochlorophyll c at 740 or 750 nm are largely due to strong pigment-pigment interactions rather than pigment-protein interactions and that the far-red bands are not necessary for energy transfer to the bacteriochlorophyll a in chlorosomes.