Photosynthetic chromatophore vesicles found in some purple bacteria constitute one of the simplest light-harvesting systems in nature. The overall architecture of chromatophore vesicles and the structural integration of vesicle function remain poorly understood despite structural information being available on individual constituent proteins. An all-atom structural model for an entire chromatophore vesicle is presented, which improves upon earlier models by taking into account the stoichiometry of core and antenna complexes determined by the absorption spectrum of intact vesicles in Rhodobacter sphaeroides, as well as the well-established curvature-inducing properties of the dimeric core complex. The absorption spectrum of low-light-adapted vesicles is shown to correspond to a light-harvesting-complex 2 to reaction center ratio of 3:1. A structural model for a vesicle consistent with this stoichiometry is developed and used in the computation of excitonic properties. Considered also is the packing density of antenna and core complexes that is high enough for efficient energy transfer and low enough for quinone diffusion from reaction centers to cytochrome bc(1) complexes.

The electrochemical reduction of CO, by derivatives of rhenium carbonyl bipyridyl at carbon cathodes in acetonitrile solution is shown to take place by different routes depending on the concentration of water in the system. In the absence of water and CO,, reduction of fac-[Re(dmbipy)(CO),CI] (dmbipy = 4,4'dimethyl-2,Z'-bipyridyl) leads first to fac-[ Re(dmbipy'-) (CO),CI] followed by elimination of CI-and partial dimerisation of t h e resultant five-co-ordinate species to give both fac-[Re(dmbipy) (CO),] and [{Re(drnbipy)(CO),},]. Further reduction gives rise to fa~-[Re(dmbipy'-)(CO)~], which is the final product. Cathodic reduction of [Re(dmbipy) (CO),CI] in the presence of CO, and absence of water leads to direct attack of CO, on fac-[Re(dmbipy'-) (CO),CI] to give [Re(dmbipy)(CO),(CO,H)] E, which can undergo further reduction to give [Re(dmbipy'-) (CO),(CO,H)]. By contrast, in the presence of H,O, the complex E can be protonated to yield [ Re(dmbipy) (CO),(CO,H,)] + which is attacked by acetonitrile to yield [Re(dmbipy) (CO),(MeCN)] + and CO, this latter complex being identified unambiguously by comparison with the species formed in the oxidation of the original rhenium complex.

The mapping of the photosynthetic membrane of Rhodobacter sphaeroides by atomic force microscopy (AFM) revealed a unique organization of arrays of dimeric reaction center-light harvesting I-PufX (RC-LH1-PufX) core complexes surrounded and interconnected by light-harvesting LH2 complexes (Bahatyrova, S., Frese, R. N., Siebert, C. A., Olsen, J. D., van der Werf, K. O., van Grondelle, R., Niederman, R. A., Bullough, P. A., Otto, C., and Hunter, C. N. (2004) Nature 430, 1058 -1062). However, membrane regions consisting solely of LH2 complexes were under-represented in these images because these small, highly curved areas of membrane rendered them difficult to image even using gentle tapping mode AFM and impossible with contact mode AFM. We report AFM imaging of membranes prepared from a mutant of R. sphaeroides, DPF2G, that synthesizes only the LH2 complexes, which assembles spherical intracytoplasmic membrane vesicles of ϳ53 nm diameter in vivo. By opening these vesicles and adsorbing them onto mica to form small, <120 nm, largely flat sheets we have been able to visualize the organization of these LH2-only membranes for the first time. The transition from highly curved vesicle to the planar sheet is accompanied by a change in the packing of the LH2 complexes such that approximately half of the complexes are raised off the mica surface by ϳ1 nm relative to the rest. This vertical displacement produces a very regular corrugated appearance of the planar membrane sheets. Analysis of the topographs was used to measure the distances and angles between the complexes. These data are used to model the organization of LH2 complexes in the original, curved membrane. The implications of this architecture for the light harvesting function and diffusion of quinones in native membranes of R. sphaeroides are discussed.The biological membrane at its simplest is a lipid bilayer that divides cellular contents from the exterior medium. Yet the bilayer performs more than a simple barrier function as it plays host to a wide variety of integral membrane proteins that perform transport, sensing, motility, biosynthesis, energy generation (in the form of ATP), as well as energy harvesting in the case of phototrophic organisms. We have structural information about the membrane proteins involved in photosynthesis, e.g. the bacterial reaction center (1), the peripheral light-harvesting complex (2), and the photosystem I (3) and photosystem II (4) supercomplexes and in transport (5), sensing (6), and electron transport (7). However, there is a need now to gain information upon the organization of these proteins within the membrane in vivo. The use of AFM 2 to directly visualize membrane proteins in situ allows us to make precise measurements of the disposition of any protein within the membrane (8) under nearly native conditions. We have demonstrated that the comparatively disordered intracytoplasmic membrane from the model photosynthetic bacterium Rhodobacter sphaeroides can be successfully imaged by AFM (9) to reveal the hidden architecture ...

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The precision placement of the desired protein components on a suitable substrate is an essential prelude to any hybrid "biochip" device, but a second and equally important condition must also be met: the retention of full biological activity. Here we demonstrate the selective binding of an optically active membrane protein, the light-harvesting LH2 complex from Rhodobacter sphaeroides, to patterned self-assembled monolayers at the micron scale and the fabrication of nanometer-scale patterns of these molecules using near-field photolithographic methods. In contrast to plasma proteins, which are reversibly adsorbed on many surfaces, the LH2 complex is readily patterned simply by spatial control of surface polarity. Near-field photolithography has yielded rows of light-harvesting complexes only 98 nm wide. Retention of the native optical properties of patterned LH2 molecules was demonstrated using in situ fluorescence emission spectroscopy.