The Structure and Thermal Motion of the B800–850 LH2 Complex from Rps. acidophila at 2.0 Å Resolution and 100 K: New Structural Features and Functionally Relevant Motions

The structure at 100 K of integral membrane light-harvesting complex II (LH2) from Rhodopseudomonas acidophila strain 10050 has been refined to 2.0 Å resolution. The electron density has been significantly improved, compared to the 2.5 Å resolution map, by high resolution data, cryo-cooling and translation, libration, screw (TLS) refinement. The electron density reveals a second carotenoid molecule, the last five C-terminal residues of the α-chain and a carboxy modified α-Met1 which forms the ligand of the B800 bacteriochlorophyll. TLS refinement has enabled the characterisation of displacements between molecules in the complex. B850 bacteriochlorophyll molecules are arranged in a ring of 18 pigments composed of nine approximate dimers. These pigments are strongly coupled and at their equilibrium positions the excited state dipole interaction energies, within and between dimers, are ∼370 cm−1 and 280 cm−1, respectively. This difference in coupling energy is similar in magnitude to changes in interaction energies arising from the pigment displacements described by TLS tensors. The displacements appear to be non-random in nature and appear to be designed to optimise the modulation of pigment energy interactions. This is the first time that LH2 pigment displacements have been quantified experimentally. The calculated energy changes indicate that there may be significant contributions to inter-pigment energy interactions from molecular displacements and these may be of importance to photosynthetic energy transfer.