Light Harvesting Complex II (LH2)

The peripheral B800-850 LH2 complex from the purple bacterium Rps. acidophila absorbs in the near infra-red at two wavelengths of around 800nm and 850 nm. The X-ray structure is composed of a cyclic arrangement of a and b peptides that pass through the membrane once. Each peptide pair is associated with 3 bacteriochlorophyll a and 2 carotenoids (rhodopin glucoside) molecules. Two B850 bacteriochlorophylls are responsible for the absorption at 850 nm and are in van der Waals contact with other pigments of the same kind while the B800 pigments are well separated and absorb at 800 nm. The whole complex is a cyclic nonamer composed of this unit. In bacteria such as Rps. molischianum the complex is an octamer while the related LH1 complex that surounds the Reaction Centre comprise a 16mer, has no B800 pigments and absorbs further into the red at ~880nm.

The structure refined at 2.0 Å, using TLS  tensors, has revealed new features not seen in the earlier maps such as: 5 additional residues on the C-terminal of the alpha chain, a second carotenoid per unit and a reasignment of the ligand of B800 to a carboxylated methionine. The TLS refinement was also able to establish the direction and magnitude of pigment motion.