Integral membrane proteins often present daunting challenges for biophysical characterization, a fundamental issue being how to select a surfactant that will optimally preserve the individual structure and functional properties of a given membrane protein. Bacterial reaction centers offer a rare opportunity to compare the properties of an integral membrane protein in different artificial lipid/surfactant environments with those in the native bilayer. Here, we demonstrate that reaction centers purified using a styrene maleic acid copolymer remain associated with a complement of native lipids and do not display the modified functional properties that typically result from detergent solubilization. Direct comparisons show that reaction centers are more stable in this copolymer/lipid environment than in a detergent micelle or even in the native membrane, suggesting a promising new route to exploitation of such photovoltaic integral membrane proteins in device applications.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms AbstractIn a quest to fabricate novel solar energy materials, the high quantum efficiency and long charge separated states of photosynthetic pigment-proteins are being exploited through their direct incorporation in bioelectronic devices. In this work, photocurrent generation by bacterial reaction center-light harvesting 1 (RC-LH1) complexes self-assembled on a nanostructured silver substrate yielded a peak photocurrent of 166 µA cm -2 under 1 sun illumination, and a maximum of over 400 µA cm -2 under 4 suns, the highest reported to date.A 2.5-fold plasmonic enhancement of light absorption per RC-LH1 complex was measured on the rough silver substrate. This plasmonic interaction was assessed using confocal fluorescence microscopy, revealing an increase of fluorescence yield and radiative rate of the RC-LH1 complexes. Nano-structuring of the silver substrate also enhanced the stability of the protein under continuous illumination by almost an order of magnitude relative to a nonstructured bulk silver control. Due to its ease of construction, increased protein loading capacity, stability and more efficient use of light, this hybrid material is an excellent candidate for further development of plasmon enhanced biosensors and bio-photovoltaic devices.3

Highlights d A cell organelle, the photosynthetic chromatophore, is modeled in atomistic detail d Segregation of protein complexes tunes chromatophore structure and function d The electrostatic environment of the organelle supports low light-adaptation d Distinct modes of quinone diffusion underpin efficient electron transfer dynamics

The Rhodobacter sphaeroides reaction centre is a relatively robust and tractable membrane protein that has potential for exploitation in technological applications, including biohybrid devices for photovoltaics and biosensing. This report assessed the usefulness of the photocurrent generated by this reaction centre adhered to a small working electrode as the basis for a biosensor for classes of herbicides used extensively for the control of weeds in major agricultural crops. Photocurrent generation was inhibited in a concentration-dependent manner by the triazides atrazine and terbutryn, but not by nitrile or phenylurea herbicides. Measurements of the effects of these herbicides on the kinetics of charge recombination in photo-oxidised reaction centres in solution showed the same selectivity of response. Titrations of reaction centre photocurrents yielded half maximal inhibitory concentrations of 208 nM and 2.1 µM for terbutryn and atrazine, respectively, with limits of detection estimated at around 8 nM and 50 nM, respectively. Photocurrent attenuation provided a direct measure of herbicide concentration, with no need for model-dependent kinetic analysis of the signal used for detection or the use of prohibitively complex instrumentation, and prospects for the use of protein engineering to develop the sensitivity and selectivity of herbicide binding by the Rba. sphaeroides reaction centre are discussed.

The cytochrome b 6 f (cytb 6 f) complex plays a central role in oxygenic photosynthesis, linking electron transfer between photosystems I and II and conserving solar energy as a transmembrane proton gradient for ATP synthesis 1-3. Electron transfer within cytb 6 f occurs via the Q-cycle, which catalyses the oxidation of plastoquinol (PQH 2) and the reduction of both plastocyanin (PC) and plastoquinone (PQ) at two separate sites via electron bifurcation 2. In higher-plants cytb 6 f also acts as a redox-sensing hub, pivotal to the regulation of light harvesting a n d c y c l i c e l e c t r o n t r a n s f e r t h at protect against metabolic and environmental stresses 3. Here we present a 3.6 Å resolution c r y o-e l e c t r o n m i c r o s c o p y (c r y o-E M) structure of the dimeric cytb 6 f complex from spinach, which reveals the structural basis for operation of the Q-cycle and its redox sensing function. The complex contains up to three natively bound PQ molecules. The first, PQ1, is bound to one cytb 6 f monomer at the PQ oxidation site (Q p) adjacent to haem b p and chlorophyll a. Two conformations of the chlorophyll a phytyl tail were resolved, one that prevents access to the Q p site and another that permits it, supporting a gating function for the chlorophyll a involved in redox sensing. PQ2 straddles the intermonomer cavity, partially obstructing the PQ reduction site (Q n) on the PQ1 side and committing the electron transfer network to turnover at the occupied Q n site in the neighbouring monomer. A conformational switch involving the haem c n propionate promotes two-electron, two-proton reduction at the Q n site and avoids formation of the reactive intermediate semiquinone. The location of a tentatively assigned third PQ molecule is consistent with a transition between the Q p and Q n sites in opposite monomers during the Q-cycle. The spinach cytb 6 f structure therefore provides new insights into how the complex fulfils its catalytic and regulatory roles in photosynthesis. Photosynthesis sustains life on Earth by converting light into chemical energy in the form of ATP and NADPH, producing oxygen as a by-product. Two light-powered electron transfer reactions at photosystems I and II (PSI and PSII) are linked via the cytb 6 f complex to form the so-called 'Z-scheme' of photosynthetic linear electron transfer (LET) 1. Cytb 6 f catalyses the rate-limiting step in the LET chain, coupling the oxidation of PQH 2 and reduction of PC and PQ to the generation of a transmembrane proton gradient (p), used by ATP synthase to make ATP 2,3. The cytb 6 f complex is analogous to the cytochrome bc 1 (cytbc 1) complex found in mitochondria 4 and anoxygenic photosynthetic bacteria 5 and both operate via the modified Q