Intramolecular Femtosecond Energy Conversion in Chlorosomes of Photosynthesizing Green Bacteria Chloroflexus aurantiacus

In the green bacterium Chloroflexus aurantiacus, photosynthesis begins with light absorption by unique light-harvesting complexes known as chlorosomes. These structures consist of ~104 molecules of bacteriochlorophyll (BChl) c, which assemble into intricate spatial configurations. Upon absorption of blue light (~460 nm) by the B band of the chlorosome, ultrafast energy conversion occurs, resulting in the excitation of the red Qy band (~740 nm). We study this process using differential (light – darkness) absorption spectroscopy with a high temporal resolution (20 fs). The characteristic conversion time is determined to be 35 fs. Energy conversion precedes the slower exciton relaxation processes in the Qy band (100–300 fs) and the energy transfer from the chlorosome to the baseplate and subsequently to the reaction center (tens of picoseconds). The physicochemical principles underlying intramolecular energy conversion in chlorosomes and the significance of this process for photosynthesis are discussed.