Triplet state quenching of bacteriochlorophyll c aggregates in a protein-free environment of a chlorosome interior

Abstract

Efficient quenching of (bacterio)chlorophyll triplet states by carotenoids prevents formation of reactive singlet oxygen in photosynthetic light-harvesting complexes. This protective process requires a close interaction between both types of pigments, which is usually ensured by a protein scaffold. Here we have studied quenching of bacteriochlorophyll c triplets in chlorosomes from the green photosynthetic bacterium Chloroflexus aurantiacus by nanosecond spectroscopy. Bacteriochlorophyll c forms aggregates in the chlorosome interior without involvement of a protein. We have observed that the triplet transfer from bacteriochlorophyll aggregates to carotenoids occurs with a transfer time of approximately 4 ns, being more than ten times slower that the estimates for the fastest triplet quenching in pigment-protein light-harvesting complexes. Nevertheless, together with aggregation-mediated shortening of excited state lifetimes, carotenoids provide efficient protection against formation of singlet oxygen in chlorosomes. Efficient triplet quenching was also observed in self-assembling, artificial light-harvesting complexes containing bacteriochlorophyll c aggregates and β-carotene. This is important for their future applications in solar energy conversion. Finally, we have studied the temperature dependence of triplet quenching in chlorosomes and determined the activation energy of the energy transfer to be about 0.08 eV. This value lies within the range estimated for pigment-protein light-harvesting complexes.