Light, harvesting

Kuhibrandt W, Wang D N and Fu]iyoshi Y 1994 Atomio model of plant light-harvesting oomplex by eleotron orystallography Nature 367 614-21... 

As an example, a series of transient hole-bnming spectra obtained with a chirp-compensated continuum probe with a light-harvesting protein is shown in figure B2.1.7 [112]. As the probe delay increases, tire initially... 

Figure B2.1.7 Transient hole-burned speetra obtained at room temperature with a tetrapyrrole-eontaining light-harvesting protein subunit, the a subunit of C-phyeoeyanin. Top fluoreseenee and absorption speetra of the sample superimposed with die speetnuu of the 80 fs pump pulses used in the experiment, whieh were obtained from an amplified CPM dye laser operating at 620 mn. Bottom absorption-diflferenee speetra obtained at a series of probe time delays.

Figure B2.1.10 Stimulated photon-echo peak-shift (3PEPS) signals. Top pulse sequence and iuterpulse delays t and T. Bottom echo signals scaimed as a fiinction of delay t at tluee different population periods T, obtained with samples of a tetrapyrrole-containing light-harvesting protein subunit, the a subunit of C-phycocyanin.

Dunn R C, Holtom G R, Mets L and Xie X S 1994 Near-field fluorescence imaging and fluorescence lifetime measurement of light harvesting complexes in intact photosynthetic membranes J. Chem. Phys. 98 3094-8... 

Tietz C, Chekhlov O, Drabenstedt A, Schuster J and Wrachtrup J 1999 Spectroscopy on single light-harvesting complexes at low temperature J. Chem. Phys. B 103 6328-33... 

Bopp M A, Jia Y, Li L, Cogdell R J and Hochstrasser R M 1997 Fluorescence and photobleaching dynamics of single light-harvesting complexes Proc. Natl Acad. Sc/. USA 94 10 630-5... 

With tlie development of femtosecond laser teclmology it has become possible to observe in resonance energy transfer some apparent manifestations of tire coupling between nuclear and electronic motions. For example in photosyntlietic preparations such as light-harvesting antennae and reaction centres [32, 46, 47 and 49] such observations are believed to result eitlier from oscillations between tire coupled excitonic levels of dimers (generally multimers), or tire nuclear motions of tire cliromophores. This is a subject tliat is still very much open to debate, and for extensive discussion we refer tire reader for example to [46, 47, 50, 51 and 55]. A simplified view of tire subject can nonetlieless be obtained from tire following semiclassical picture. 

Fetisova Z G, Borisov A Y and Fok M V 1985 Analysis of structure-function correlations in light-harvesting photosynthetic antenna—structure optimization parameters J. Theoret. Biol. 112 41-75... 

Chaohisvilis M and Sundstrom V 1996 Femtoseoond vibrational dynamios and relaxation in the oore light-harvesting oomplex of photosynthetio purple baoteria Chem. Rhys. Lett. 261 165-74... 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

Given the difficulty of obtaining three-dimensional crystals of membrane proteins, it is not surprising that the electron microscope technique is now widely used to study large membrane-bound complexes such as the acetylcholine receptor, rhodopsin, ion pumps, gap junctions, water channels and light-harvesting complexes, which crystallize in two dimensions. 

Antenna pigment proteins assemble into multimeric light-harvesting particles... 

Figure 12.17 Computer-generated diagram of the stmcture of light-harvesting complex LH2 from Rhodopseudomonas acidophila. Nine a chains (gray) and nine p chains Bight blue) form two rings of transmembrane helices between which are bound nine carotenoids (yellow) and 27 bacteriochlorophyll molecules (red, green and dark blue). (Courtesy of M.Z. Papiz.)...

Chlorophyll molecules form circular rings in the light-harvesting complex LH2... 

Figure 12.18 Ribbon diagram showing the a (red) and the P (blue) chains of the light-harvesting complex LH2. Each chain forms one transmembrane a helix, which contains a histidine residue that binds to the Mg atom of one bacteriochlorophyll molecule. (Adapted from G. McDermott et al.. Nature 374 517-521, 1995.)...

TTie reaction center is surrounded by a ring of 16 antenna proteins of the light-harvesting complex LHl... 

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

Figure 12.19 Schematic diagrams illustrating the arrangement of hacteriochlorophyll molecules in the light-harvesting complex LH2, viewed from the periplasmic space, (a) Eighteen hacteriochlorophyll molecules (green] are hound between the two rings of a (red) and p (blue) chains. The planes of these molecules are oriented perpendicular to the plane of the membrane and the molecules are bound close to the periplasmic space, (b) Nine hacteriochlorophyll molecules (green) are bound between the p chains (blue) with their planes oriented parallel to the plane of the membrane. These molecules are bound in the middle of the membrane.

Karrasch, S., Bullough, RA., Ghosh, R. 8.5-A projection map of the light-harvesting complex I from Rhodospir-illum rubrum reveals a ring composed of 16 subunits. EMBO J. 14 631-638, 1995. 

Koepke, J., et al. The crystal structure of the light-harvesting complex II (B800-850) from Rhodospirillum molis-chianum. Structure 4 581-597, 1996. 

Kiihlbrandt, W., Wang, D.A., Fujiyoshi, Y. Atomic model of the plant light-harvesting complex. Nature 367 614-621, 1994. 

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