The influence of symmetry on the electronic structure of the photosynthetic pigment-protein complexes from purple bacteria

Abstract

The primary reactions of purple bacterial photosynthesis take place in two pigment-protein complexes, the peripheral LH2 complex and the core RC- LH1 complex. In order to understand any type of excitation-energy transfer in the LH system detailed knowledge about the correlation between the geometrical structure and the nature of the electronically excited states is crucial. The interplay between the geometrical arrangement of the pigments and the transition probabilities of the various exciton states leads to key spectral features, such as narrow lines, that are clearly visible with single-molecule spectroscopy but are averaged out in conventional ensemble experiments. Combining low-temperature single-molecule spectroscopy with numerical simulations has allowed us to achieve a refined structural model for the bacteriochlorophyll a (BChl a) pigment arrangement in RC-LH1 core complexes of Rps. palustris. The experimental data are consistent with an equidistant arrangement of 15 BChl a dimers on an ellipse, where each dimer has been taken homologeous to those from the B850 pigment pool of LH2 from Rps. acidophila.