AQUEOUS AGGREGATES OF BACTERIOCHLOROPHYLL c AS A MODEL FOR PIGMENT ORGANIZATION IN CHLOROSOMES

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Abstract
Chlorosomes isolated from Chloroflexus aurantiacus were extracted with chloroform/methanol. The extract contained bacteriochlorophylls c and a and lipids, but was devoid of proteins. This crude extract spontaneously formed aggregates when a methanol solution was dispersed in aqueous buffer.The aggregates could be sedimented by ultracentrifugation and appeared in electron micrographs as stain‐excluding bodies with diameters between 70 and 170 nm. The absorption spectrum is remarkably similar to that of intact chlorosomes with an absorption maximum of bacteriochlorophyll c at around 740 nm. The circular dichroism spectrum of the aggregate is also very similar to that of intact chlorosomes. A conservative (±) band centered at 740 nm confirms the highly aggregated state of bacteriochlorophyll c in both systems.Steady‐state fluorescence studies showed that in the aggregate energy‐transfer from bacteriochlorophyll c to a component emitting at 830 nm took place. When the aggregate was suspended in buffer saturated with 1‐hexanol the 740 nm form of bacteriochlorophyll c was reversibly converted to a form with spectral properties resembling the monomer absorbing at 670 nm but still in an aggregated state. This form of bacteriochlorophyll c showed no circular dichroism signal.

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