Femtosecond exciton relaxation in chlorosomes of the photosynthetic green bacterium <i>Chloroflexus aurantiacus</i>

Abstract

In the green bacteria Chloroflexus (Cfx.) aurantiacus, the process of photosynthesis begins with the absorption of light by chlorosomes, peripheral antennas consisting of thousands of bacteriochlorophyll c (BChl c) molecules combined into oligomeric structures. In this case, excited states are formed in BChl c, the energy of which migrates along the chlorosome towards the baseplate and further to the reaction center, where the primary charge separation occurs. Energy migration is accompanied by nonradiative electronic transitions between numerous exciton states, that is exciton relaxation. In this work, we studied the dynamics of exciton relaxation in Cfx. aurantiacus chlorosomes using difference femtosecond spectroscopy at cryogenic temperature (80 K). Chlorosomes were excited by 20 fs light pulses at wavelengths from 660 to 750 nm, and the difference (light-dark) kinetics were measured at a wavelength of 755 nm. Mathematical analysis of the data obtained revealed the kinetic components with characteristic time constants of 140, 220, and 320 fs responsible for the exciton relaxation. As the excitation wavelength decreased, the number of components and their relative contribution increased. Theoretical modeling of the results obtained was carried out on the base of the cylindrical model of BChl c aggregates. Nonradiative transitions between groups of exciton bands were ascribed by the system of kinetic equations. The model in which the energetic and structural disorder was taken into account turned out to be the most adequate.