Generation and quenching of singlet molecular oxygen by aggregated bacteriochlorophyll d in model systems and chlorosomes.

Both photogeneration and quenching of singlet oxygen by monomeric and aggregated (dimeric and oligomeric) molecules of bacteriochlorophyll (BChl) d have been studied in solution and in chlorosomes isolated from the green photosynthetic bacterium Chlorobium vibrioforme f. thiosulfatophilum. The yield of singlet-oxygen photogeneration by pigment dimers was about 6 times less than for monomers. Singlet oxygen formation was not observed in oligomer-containing solutions or in chlorosomes. To estimate the efficiency of singlet oxygen quenching an effective rate constant for (1)O2 quenching by BChl molecules (kq (M)) was determined using the Stern-Volmer equation and the total concentration of BChl d in the samples. In solutions containing only monomeric BChl, the kq (M) values coincide with the real values for (1)O2 quenching rate constants by BChl molecules. Aggregation weakly influenced the kq (M) values in pigment solutions. In chlorosomes (which contain both BChl and carotenoids) the kq (M) value was less than in solutions of BChl alone and much less than in acetone extracts from chlorosomes. Thus (1)O2 quenching by BChl and carotenoids is much less efficient in chlorosomes than in solution and is likely caused primarily by BChl molecules which are close to the surface of the large chlorosome particles. The data allow a general conclusion that monomeric and dimeric chlorophyll molecules are the most likely sources of (1)O2 formation in photosynthetic systems and excitation energy trapping by the long wavelength aggregates as well as (1)O2 physical quenching by monomeric and aggregated chlorophyll can be considered as parts of the protective system against singlet oxygen formation.