Single Molecule Spectroscopy of Pigment Protein Complexes from Purple Bacteria

Abstract

In bacterial photosynthesis, light energy is absorbed by a network of antenna pigment proteins and efficiently transferred to the photochemical reaction center where a charge separation takes place providing the free energy for subsequent chemical reactions. Most photosynthetic purple bacteria contain two types of antenna complexes: light-harvesting complex 1 (LH1) and light-harvesting complex 2 (LH2). It is known that the photochemical reaction center is closely associated with the LH1 complex while the LH2 complexes are arranged around the perimeter of LH1 in a two-dimensional structure. The transfer of energy from LH2 to LH1 and subsequently to the reaction center occurs in vivo on a time scale of 30–40 ps, i.e., very fast compared to the decay of an isolated LH2 which has a fluorescence lifetime of about 1 ns. As yet, there is no consensus about the details of the mechanisms of the energy-transfer process, and the full three-dimensional structure of the whole photosynthetic unit is still unknown. The great difficulty encountered when determining the various parameters that play a role in the description of the electronic structure of light-harvesting complexes and the process of energy transfer, is the fact that the optical absorption lines are inhomogeneously broadened as a result of heterogeneity in the ensemble of absorbing pigments. To circumvent this problem we have applied single-molecule detection schemes to study the pigment-protein complexes individually thereby avoiding ensemble averaging. Here we present an overview of our work on LH1 and LH2 from Rhodopseudomonas acidophila.