Self-assembly and Optical Spectroscopy of Bacteriochlorophyll Aggregates Containing Quinones

Bacteriochlorophyll (BChl) c is the main constituent of chlorosomes, the light harvesting complexes of green photosynthetic bacteria, where it is found in the form of aggregates. Similar aggregates can be prepared also in vitro. In this study we show that the aggregation of BChl c in aqueous buffers can be induced by quinones possessing a hydrophobic tail. Such quinones are naturally present in chlorosomes and are involved in a redox-dependent excitation quenching mechanism, which may protect the cells against toxic substances produced as a consequence of charge separation under oxygenic conditions. BChl aggregates prepared with quinones also exhibit such an excitation quenching, as determined from fluorescence spectra. The amplitude of the quenching is proportional to concentration of quinones and is similar as for BChl aggregates prepared with quinones and lipids.