Exciton self-trapping in bacterial antennas promotes light harvesting efficiency

Abstract

There is considerable variability in the transition energies of individual antenna complexes from purple photosynthetic bacteria, raising the question of how rapid energy transfer between these complexes is accomplished. Here, based on steady state and time-resolved absorption and emission studies of peripheral LH2 antenna complexes isolated from the membrane of the purple bacterium Rhodobacter sphaeroides, we present evidence that the absorbing and emitting states of this complex are qualitatively different. The free exciton created by light absorption dynamically localizes within ~100 fs by coupling with nuclear vibrations to form a self-trapped exciton. The accompanying spatial deformation covers ~20% of the complex circumference at low temperature. This self-trapping, the first in its kind observed in biological systems, results in a broad fluorescence spectrum and considerably improves energy resonance between heterogeneous LH2 antenna complexes. Furthermore, the red shift of the LH2 antenna spectra induced by self-trapping improves overlap with the absorption spectra of the core LH1 antennas surrounding the reaction centre. The very weak temperature dependence of the antenna spectra implies that the observed phenomenon is also relevant at physiological temperatures. The self-trapping thus has an important biological function in promoting efficient light harvesting in spectrally heterogeneous bacterial photosynthetic units.