X-ray structure of the light-harvesting complex LH2 from Rhodospirillum molischianum. Click on the images at right to switch between views of the molecule. Click on the "Hide" button to see only the pigment molecules. (a) View perpendicular to the photosynthetic membrane showing that the eight α subunits (blue, 56 residues) and the eight β subunits (magenta, 45 residues), drawn in worm form, are arranged in two concentric eightfold symmetric rings. Thirty-two bacteriochlorophyll a (BChl a, green with Mg²⁺ ions represented by white spheres) and eight lycopene (a carotenoid, yellow) molecules are sandwiched between the protein rings. (b) View from the plane of the membrane, using the same colors as in the top image, in which the α helical portions of the proteins are represented by cylinders. Note that 8 of the BChl a molecules are bound near the top of the complex with their ring systems nearly parallel to the plane of the membrane, whereas the remaining 16 BChl a molecules are bound near the bottom of the complex with their ring systems approximately perpendicular to the plane of the membrane. This arrangement, together with that of the lycopene molecules, presumably optimizes the light-absorbing and excitation-transmitting capability of this antenna system. [X-ray structure by Juergen Koepke and Hartmut Michel, Max-Planck Institut fuer Biochemie, Frankfurt, Germany. PDBid 1LGH.]