LH2 light-harvesting complex

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.)...

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.

Fig. 1. Light harvesting complex LH2 from Rps. acidophila. a) Absorption spectrum, b) Standard model of the energy flow rates upon excitation of Car S2 [3].

Figure 23-27 Illustration of proposed exciton transfer of the energy of light absorbed by bacteriochlorophyll a of purple bacteria. Energy absorbed by the light harvesting complex LH2 is transferred in steps to another LH2, to LH1 and to the reaction center. The short lines within the circles represent the edges of the BChla chromophores. After Kiihlbrandt300 with permission.

Prince SM, Howard TD, Myles DA, Wilkinson C, Papiz MZ, Freer AA, Cogdell RJ, Isaacs NW. Detergent structure in crystals of the integral membrane light-harvesting complex LH2 from Rhodopseudomonas acidophila strain 10050. J. Mol. Biol. 2003 326 307-315. 

Scheuring S, Seguin J, Marco S, Levy D, Breyton C, Robert B, Rigaud JL. AFM characterization of tilt and intrinsic flexibility of Rhodobacter sphaeroides light harvesting complex 2 (LH2). J. Mol. Biol. 2003 325 569-580. 

K. Timpmann, M. Ratsep, C.N. Hunter, A. Freiberg, Emitting excitonic polaron states in Core LHl and peripheral LH2 bacterial light-harvesting complexes. J. Phys. Chem. B 108, 10581-10588 (2004)... 

Low-temperature N and CPMAS NMR has been used to analyze BChl-histidine interactions and the electronic structure of histidine residues in the light-harvesting complex II (LH2) of Rhodopseudomonas acidophila. Comparison of the 2D MAS NMR homonuclear ( C- C) dipolar correlation spectrum of [ C(6), N(3)]-histidines in the LH2 complex with model systems in the solid state reveals two different classes of electronic structures from the... 

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