Changes in bacteriochlorophyll c organization during acid treatment of chlorosomes from Chlorobium tepidum

We have investigated changes in the organization of bacteriochlorophyll c (BChl c) in chlorosomes isolated from the green sulfur bacterium Chlorobium tepidum during the formation of bacteriopheophytin and bacteriopheophorbide in acidic media. The reaction was much slower than that observed with BChl dissolved in methanol, suggesting that the aggregation of BChl or the presence of the chlorosome envelope constitutes a barrier to the reaction with protons in the aqueous phase. In most cases the first two-thirds of the reaction showed time courses that were close to linear. Simultaneously with the pheophytinization process we observed a red-shift of the Qy band of the remaining aggregated BChl c reaching a maximum extent of 9 nm. Analysis of the spectral changes provided evidence at least for two spectrally distinct pools of aggregated BChl c with different rates of reaction with acid. An HPLC analysis showed that there were no changes in the distribution of the four major homologs of BChl c remaining in chlorosomes during the course of acid treatment, at least up to the time when two-thirds had been converted to pheophytin. This suggests that the homologs of BChl c are uniformly distributed within the chlorosome.