Figure 1. The upper half shows the molecular structure of Bchla, giving the positions of the Q_x and Q_y dipoles and the numbering scheme used in this article. The lower half shows the absorption spectrum of an LH2 complex. The three bands around 500 nm are due to the carotenoid, the band at 590 nm comes from Bchla absorbing through the Q_x dipole and the bands at 800 and 850 nm are from Bchla absorption through the Q_y dipoles. The spectrum of an LH3 complex is similar except the band at 850 nm is shifted to 820 nm. The spectrum of the LH1 complex has a single band at about 870 nm instead of the two bands labelled B800 and B850.

Figure 2. Two views of the light-harvesting complex from Rps. acidophila. (a) The nonameric complex viewed from the surface of the membrane. The traces of the protein chains are coloured white, B800 Bchla green, B850 Bchla red and carotenoid yellow. (b) The broad ribbon trace for the helices indicate approximately the hydrophobic region of the membrane. Reprinted by permission from McDermott et al. (1995) Nature 374: 517–521 © 1995 Macmillan Magazines Ltd.

Figure 3. The arrangement of the Bchla molecules viewed from the surfaces of the cell membrane. (a) The bacteriochlorin rings of the eighteen B850 molecules are perpendicular to the membrane surface and form a complete ring enclosed by the helices of the -apoproteins (yellow) and -apoproteins (green). (b) Viewed from the opposite direction, the bacteriochlorin rings of the B800 molecules are parallel to the membrane surface and lie between the -apoproteins. They are coordinated to the N-terminal residues of the -apoproteins. Figure 3(a) reprinted by permission from McDermott et al. (1995) Nature 374: 517–521 © 1995 Macmillan Magazines Ltd.

Figure 4. The arrangements of the pigment molecules showing the packing of the bacteriochlorin rings and intertwining of the phytyl chains and carotenoids. The B800 Bchla are coloured green; B850 coordinated to the -apoprotein are red; B850 coordinated to the -apoprotein are orange; carotenoids are yellow.

Figure 5. The proposed arrangement of light-harvesting complexes and reaction centre in the membrane. The view is a slice through the membrane across the diameters of adjacent LH2 and LH1 complexes. The shading within the proteins shows the positions of the Bchla pigments. The directions and times for energy transfer are shown. RC = reaction centre.