The Electronic Structure of Single Photosynthetic Pigment-Protein Complexes

The development of optical techniques to study single molecules in the condensed phase [1] opened the way for the investigation of molecular interactions on a truly microscopic scale. These single-molecule measurements reveal the distribution of molecular properties in inhomogeneous systems, properties that are normally obscured by ensemble averaging. In the study presented here, single-molecule techniques are used to investigate the electronic structure of antenna complexes of photosynthetic purple bacteria. The initial event in bacterial photosynthesis is the absorption of a photon by a light-harvesting antenna system, which is followed by a rapid and highly efficient transfer of the energy to the reaction center (RC), where charge separation takes place and the energy becomes available as chemical energy. In most purple bacteria, the photosynthetic membranes contain two types of light-harvesting (LH) complexes, the LH1 and the LH2 complex. LH1 is known to directly surround the RC, whereas LH2 is not in direct contact with the RC but transfers the energy to the RC via the LH1 complex [2, 3]. The high-resolution X-ray structure of the LH2 complex of Rps. acidophila [4], along with lower-resolution structural information for LH1 [5], showed a remarkable symmetry in the arrangement of the light-absorbing pigments in their protein matrix. This LH2 complex consists of nine copies of a pair of proteins (α and β) arranged in a ring structure with C9 symmetry, where each αβ unit binds three BChl a and (presumably) two carotenoid molecules. The arrangement of the pigments is indicated in Fig. 3.1, where only the bacteriochlorin rings of the BChl a molecules are shown for clarity. A striking feature of the organization of the 27 BChl a molecules is their separation in two parallel rings. One ring consists of a group of 18 closely packed BChl a molecules, with their bacteriochlorin planes parallel to the symmetry axis, absorbing at 850 nm (B850). The other ring comprises nine well-separated BChl a molecules absorbing at 800 nm (B800). The molecules in this B800 ring have their bacteriochlorin planes perpendicular to the symmetry axis of the complex. Upon excitation, energy is transferred from B800 to B850 molecules in 1 to 2 ps [6, 7, 8], while energy transfer among the B850 molecules is an order of magnitude faster [9, 10, 11]. The transfer of energy from LH2 to LH1 and subsequently to the RC occurs in vivo on a time scale of 30–40 ps, i.e., very fast compared to the fluorescence decay of B850 in isolated LH2, which has a time constant of 1 ns.