Photosynthetic reaction center functionalized nano-composite films: effective strategies for probing and exploiting the photo-induced electron transfer of photosensitive membrane protein

Conversion of light into electricity in a semi-synthetic system based on photosynthetic bacterial chromatophores

Chromatophores (Chr) from photosynthetic nonsulfur purple bacterium Rhodobacter sphaeroides immobilized onto a Millipore membrane filter (MF) and sandwiched between two semiconductor indium tin oxide (ITO) electrodes (termed ITO|Chr - MF|ITO) have been used to measure voltage (ΔV) induced by continuous illumination. The maximum ΔV was detected in the presence of ascorbate / N,N,N'N'-tetramethyl-p-phenylenediamine couple, coenzyme UQ₀, disaccaride trehalose and antimycin A, an inhibitor of cytochrome bc₁ complex. In doing so, the light-induced electron transfer in the reaction centers was the major source of photovoltages. The stability of the voltage signal upon prolonged irradiation (>1 h) may be due to the maintenance of a conformation that is optimal for the functioning of integral protein complexes and stabilization of lipid bilayer membranes in the presence of trehalose. Retaining ∼70 % of the original photovoltage performance on the 30th day of storage at 23 °C in the dark under air was achieved after re-injection of fresh buffer (∼40 μL) containing redox mediators into the ITO|Chr - MF|ITO system. The approach we use is easy and can be extended to other biological intact systems (cells, thylakoid membranes) capable of converting energy of light.

Photosynthetic Nanomaterial Hybrids for Bioelectricity and Renewable Energy Systems

Harvesting solar energy in the form of electricity from the photosynthesis of plants, algal cells, and bacteria has been researched as the most environment-friendly renewable energy technology in the last decade. The primary challenge has been the engineering of electrochemical interfacing with photosynthetic apparatuses, organelles, or whole cells. However, with the aid of low-dimensional nanomaterials, there have been many advances, including enhanced photon absorption, increased generation of photosynthetic electrons (PEs), and more efficient transfer of PEs to electrodes. These advances have demonstrated the possibility for the technology to advance to a new level. In this article, the fundamentals of photosynthesis are introduced. How PE harvesting systems have improved concerning solar energy absorption, PE production, and PE collection by electrodes is discussed. The review focuses on how different kinds of nanomaterials are applied and function in interfacing with photosynthetic materials for enhanced PE harvesting. Finally, the review analyzes how the performance of PE harvesting and stand-alone systems have evolved so far and its future prospects.

Ganoderma spore powder contains little triterpenoids

Background

Ganoderma spore is a minuscule germ cell ejected from Ganoderma gills during its growth maturity period, it has been considered with high exploitable potential in health-care products manufacture.

Methods

After testing sporoderm-broken rate, the triterpenoids in 12 batches of broken and unbroken Ganoderma spore powder (GSP) samples were compared with Ganoderma lucidum fruiting body (GL) by high performance thin-layer chromatography (HPTLC) and further verified by liquid chromatography coupled with electrospray ionization quadrupole-time-of-flight mass spectrometry (LC–QTOF–MS); meanwhile, the dissolution of triterpenoids after bionic extraction was also investigated by HPTLC.

Results

The sporoderm-broken rate of all the broken GSP samples was over 85%. The relative peak area of triterpenoids in GSP samples were lower than 50% of that in fruiting body, and the dissolution of triterpenoids in artificial gastrointestinal fluid was lower than in methanol.

Conclusions

This study demonstrated that there were little triterpenoids in GSP. Triterpenoids in GSP also seldom be dissolved in artificial gastrointestinal fluid.

Primary electron transfer in Rhodobacter sphaeroides R-26 reaction centers under dehydration conditions

The photoinduced charge separation in Q_B-depleted reaction centers (RCs) from Rhodobacter sphaeroides R-26 in solid air-dried and vacuum-dried (~10^-2 Torr) films, obtained in the presence of detergent n-dodecyl-β-D-maltoside (DM), is characterized using ultrafast transient absorption spectroscopy. It is shown that drying of RC-DM complexes is accompanied by reversible blue shifts of the ground-state absorption bands of the pigment ensemble, which suggest that no dehydration-induced structural destruction of RCs occurs in both types of films. In air-dried films, electron transfer from the excited primary electron donor P^⁎ to the photoactive bacteriopheophytin H_A proceeds in 4.7 ps to form the P⁺H_A^- state with essentially 100% yield. P⁺H_A^- decays in 260 ps both by electron transfer to the primary quinone Q_A to give the state P⁺Q_A^- (87% yield) and by charge recombination to the ground state (13% yield). In vacuum-dried films, P^⁎ decay is characterized by two kinetic components with time constants of 4.1 and 46 ps in a proportion of ~55%/45%, and P⁺H_A^- decays about 2-fold slower (462 ps) than in air-dried films. Deactivation of both P^⁎ and P⁺H_A^- to the ground state effectively competes with the corresponding forward electron-transfer reactions in vacuum-dried RCs, reducing the yield of P⁺Q_A^- to 68%. The results are compared with the data obtained for fully hydrated RCs in solution and are discussed in terms of the presence in the RC complexes of different water molecules, the removal/displacement of which affects spectral properties of pigment cofactors and rates and yields of the electron-transfer reactions.

Spectral and Photochemical Properties of Rhodobacter sphaeroides R-26 Reaction Center Films in Vacuum

Using absorption spectroscopy in the visible/near-IR and mid-IR regions, spectral and photochemical properties of isolated reaction centers (RCs) from Rhodobacter sphaeroides R-26 were studied in dried films on the inorganic support surface (quartz or CaF₂ plates) under vacuum dehydration conditions (10^-2 or 7·10^-5 mm Hg). Three detergents, N,N-dimethyldodecylamine N-oxide (LDAO), Triton X-100 (TX100), and n-dodecyl-β-D-maltoside (DM), were tested for their ability to stabilize the RC-detergent complexes in the vacuum-dried state. It was shown that in the presence of LDAO, RC complexes underwent destruction in vacuum. In contrast, DM provided an environment that minimized irreversible disruptive changes in the RCs in vacuum. The effects of vacuum dehydration on the RC-DM films included a small increase in the content of α-helices in the RC protein, a short-wavelength reversible shift in the optical transitions of pigments, and minor changes in the electronic structure of the P⁺ dimer. The films retained their photochemical activity upon excitation with high-intensity light (200 mW/cm²). TX100 also helped to maintain spectral and functional properties of the RCs in vacuum; however, in this case, the stabilizing effect was less pronounced than in the presence of DM, especially, at high detergent concentrations. The results are discussed within the framework of a model suggesting that the detergent-protein interactions and the properties of detergent micelles play a dominant role in maintaining the structure of the RCs upon vacuum dehydration of the RC complexes. The obtained data can be useful for developing hybrid photoconverting systems based on bacterial RCs.

Modelling of the cathodic and anodic photocurrents from Rhodobacter sphaeroides reaction centres immobilized on titanium dioxide

As one of a number of new technologies for the harnessing of solar energy, there is interest in the development of photoelectrochemical cells based on reaction centres (RCs) from photosynthetic organisms such as the bacterium Rhodobacter (Rba.) sphaeroides. The cell architecture explored in this report is similar to that of a dye-sensitized solar cell but with delivery of electrons to a mesoporous layer of TiO₂ by natural pigment-protein complexes rather than an artificial dye. Rba. sphaeroides RCs were bound to the deposited TiO₂ via an engineered extramembrane peptide tag. Using TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) as an electrolyte, these biohybrid photoactive electrodes produced an output that was the net product of cathodic and anodic photocurrents. To explain the observed photocurrents, a kinetic model is proposed that includes (1) an anodic current attributed to injection of electrons from the triplet state of the RC primary electron donor (P^T) to the TiO₂ conduction band, (2) a cathodic current attributed to reduction of the photooxidized RC primary electron donor (P⁺) by surface states of the TiO₂ and (3) transient cathodic and anodic current spikes due to oxidation/reduction of TMPD/TMPD⁺ at the conductive glass (FTO) substrate. This model explains the origin of the photocurrent spikes that appear in this system after turning illumination on or off, the reason for the appearance of net positive or negative stable photocurrents depending on experimental conditions, and the overall efficiency of the constructed cell. The model may be a used as a guide for improvement of the photocurrent efficiency of the presented system as well as, after appropriate adjustments, other biohybrid photoelectrodes.

A ZnO nanowire bio-hybrid solar cell

Harvesting solar energy as a carbon free source can be a promising solution to the energy crisis and environmental pollution. Biophotovoltaics seek to mimic photosynthesis to harvest solar energy and to take advantage of the low material costs, negative carbon footprint, and material abundance. In the current study, we report on a combination of zinc oxide (ZnO) nanowires with monolayers of photosynthetic reaction centers which are self-assembled, via a cytochrome c linker, as photoactive electrode. In a three-probe biophotovoltaics cell, a photocurrent density of 5.5 μA cm^-2 and photovoltage of 36 mV was achieved, using methyl viologen as a redox mediator in the electrolyte. Using ferrocene as a redox mediator a transient photocurrent density of 8.0 μA cm^-2 was obtained, which stabilized at 6.4 μA cm^-2 after 20 s. In-depth electronic structure characterization using photoemission spectroscopy in conjunction with electrochemical analysis suggests that the fabricated photoactive electrode can provide a proper electronic path for electron transport all the way from the conduction band of the ZnO nanowires, through the protein linker to the RC, and ultimately via redox mediator to the counter electrode.

Supramolecular nanostructures based on bacterial reaction center proteins and quantum dots

Design of the nanostructures based on membrane proteins (the key functional elements of biomembranes) and colloid nanoparticles is a fascinating field at the interface of biochemistry and colloids, nanotechnology and biomedicine. The review discusses the main achievements in the field of ultrathin films prepared from bacterial reaction center proteins and light-harvesting complexes, as well as these complexes tagged with quantum dots. The principles of preparation of these thin films and their structure and properties at different interfaces are described; as well as their characteristics estimated using a combination of the modern interfacial techniques (absorption and fluorescence spectroscopy, atomic force and Brewster angle microscopy, etc.) are discussed. Further approaches to develop the nanostructures based on the membrane proteins and quantum dots are suggested. These supramolecular nanostructures are promising prototypes of the materials for photovoltaic, optoelectronic and biosensing applications.

Large photocurrent response and external quantum efficiency in biophotoelectrochemical cells incorporating reaction center plus light harvesting complexes

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high ratio of photogenerated electrons to absorbed photons and long recombination time of generated charges. In this work, photoactive electrodes were prepared from a bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is encircled by the LH1 antenna, to increase light capture. A simple immobilization method was used to prepare RC-LH1 photoactive layer. Herein, we demonstrate that the combination of pretreatment of the RC-LH1 protein complexes with quinone and the immobilization method results in biophotoelectrochemical cells with a large peak transient photocurrent density and photocurrent response of 7.1 and 3.5 μA cm(-2), respectively. The current study with monochromatic excitation showed maximum external quantum efficiency (EQE) and photocurrent density of 0.21% and 2 μA cm(-2), respectively, with illumination power of ∼6 mW cm(-2) at ∼875 nm, under ambient conditions. This work provides new directions to higher performance biophotoelectrochemical cells as well as possibly other applications of this broadly functional photoactive material.

Demonstration of asymmetric electron conduction in pseudosymmetrical photosynthetic reaction centre proteins in an electrical circuit

Photosynthetic reaction centres show promise for biomolecular electronics as nanoscale solar-powered batteries and molecular diodes that are amenable to atomic-level re-engineering. In this work the mechanism of electron conduction across the highly tractable Rhodobacter sphaeroides reaction centre is characterized by conductive atomic force microscopy. We find, using engineered proteins of known structure, that only one of the two cofactor wires connecting the positive and negative termini of this reaction centre is capable of conducting unidirectional current under a suitably oriented bias, irrespective of the magnitude of the bias or the applied force at the tunnelling junction. This behaviour, strong functional asymmetry in a largely symmetrical protein–cofactor matrix, recapitulates the strong functional asymmetry characteristic of natural photochemical charge separation, but it is surprising given that the stimulus for electron flow is simply an externally applied bias. Reasons for the electrical resistance displayed by the so-called B-wire of cofactors are explored.

Photosynthetic reaction centres have been proposed for applications in bioelectronics. Here, the authors examine electron transport through the reaction centre from R. sphaeroides using conductive AFM, observing asymmetric conductance along only one cofactor wire under an applied bias.

Photosynthetic machineries in nano-systems

Photosynthetic reaction centres are membrane-spanning proteins, found in several classes of autotroph organisms, where a photoinduced charge separation and stabilization takes place with a quantum efficiency close to unity. The protein remains stable and fully functional also when extracted and purified in detergents thereby biotechnological applications are possible, for example, assembling it in nano-structures or in optoelectronic systems. Several types of bionanocomposite materials have been assembled by using reaction centres and different carrier matrices for different purposes in the field of light energy conversion (e.g., photovoltaics) or biosensing (e.g., for specific detection of pesticides). In this review we will summarize the current status of knowledge, the kinds of applications available and the difficulties to be overcome in the different applications. We will also show possible research directions for the close future in this specific field.

Photoelectrochemical bioanalysis: the state of the art

This review provides a panoramic snapshot of the state of the art in the dynamically developing field of photoelectrochemical bioanalysis.

Photosynthetic reaction centers/ITO hybrid nanostructure

Photosynthetic reaction center proteins purified from Rhodobacter sphaeroides purple bacterium were deposited on the surface of indium tin oxide (ITO), a transparent conductive oxide, and the photochemical/-physical properties of the composite were investigated. The kinetics of the light induced absorption change indicated that the RC was active in the composite and there was an interaction between the protein cofactors and the ITO. The electrochromic response of the bacteriopheophytine absorption at 771 nm showed an increased electric field perturbation around this chromophore on the surface of ITO compared to the one measured in solution. This absorption change is associated with the charge-compensating relaxation events inside the protein. Similar life time, but smaller magnitude of this absorption change was measured on the surface of borosilicate glass. The light induced change in the conductivity of the composite as a function of the concentration showed the typical sigmoid saturation characteristics unlike if the photochemically inactive chlorophyll was layered on the ITO. In this later case the light induced change in the conductivity was oppositely proportional to the chlorophyll concentration due to the thermal dissipation of the excitation energy. The sensitivity of the measurement is very high; few picomole RC can change the light induced resistance of the composite.

Porous silicon/photosynthetic reaction center hybrid nanostructure

The purified photosynthetic reaction center protein (RC) from Rhodobacter sphaeroides R-26 purple bacteria was bound to porous silicon microcavities (PSiMc) either through silane-glutaraldehyde (GTA) chemistry or via a noncovalent peptide cross-linker. The characteristic resonance mode in the microcavity reflectivity spectrum red shifted by several nanometers upon RC binding, indicating the protein infiltration into the porous silicon (PSi) photonic structure. Flash photolysis experiments confirmed the photochemical activity of RC after its binding to the solid substrate. The kinetic components of the intraprotein charge recombination were considerably faster (τ(fast) = 14 (±9) ms, τ(slow) = 230 (±28) ms with the RC bound through the GTA cross-linker and only τ(fast) = 27 (±3) ms through peptide coating) than in solution (τ(fast) = 120 (±3) ms, τ(slow) = 1387 (±2) ms), indicating the effect of the PSi surface on the light-induced electron transfer in the protein. The PSi/RC complex was found to oxidize the externally added electron donor, mammalian cytochrome c, and the cytochrome oxidation was blocked by the competitive RC inhibitor, terbutryne. This fact indicates that the specific surface binding sites on the PSi-bound RC are still accessible to external cofactors and an electronic interaction with redox components in the aqueous environment is possible. This new type of biophotonic material is considered to be an excellent model for new generation applications at the interface of silicon-based electronics and biological redox systems designed by nature.

Enhanced photocurrent generation by photosynthetic bacterial reaction centers through molecular relays, light-harvesting complexes, and direct protein-gold interactions

The utilization of proteins as nanodevices for solar cells, bioelectronics, and sensors generally necessitates the transfer of electrons to or from a conducting material. Here we report on efforts to maximize photocurrent generation by bacterial photosynthetic reaction center pigment-protein complexes (RCs) interfaced with a metal electrode. The possibility of adhering RCs to a bare gold electrode was investigated with a view to minimizing the distance for electron tunneling between the protein-embedded electron-transfer cofactors and the metal surface. Substantial photocurrents were achieved despite the absence of coating layers on the electrode or engineered linkers to achieve the oriented deposition of RCs on the surface. Comparison with SAM-covered gold electrodes indicating enhanced photocurrent densities was achieved because of the absence of an insulating layer between the photoactive pigments and the metal. Utilizing RCs surrounded by light-harvesting 1 complex resulted in higher photocurrents, surprisingly not due to enhanced photoabsorption but likely due to better surface coverage of uniformly oriented RC-LH1 complexes and the presence of a tetraheme cytochrome that could act as a connecting wire. The introduction of cytochrome-c (cyt-c) as a molecular relay also produced increases in current, probably by intercalating between the adhered RCs or RC-LH1 complexes and the electrode to mediate electron transfer. Varying the order in which components were introduced to the electrode indicated that dynamic rearrangements of RCs and cyt-c occurred at the bare metal surface. An upper limit for current generation could not be detected within the range of the illumination power available, with the maximum current density achieved by RC-LH1 complexes being on the order of 25 μA/cm(2). High currents could be generated consecutively for several hours or days under ambient conditions.

Prokaryotic gene clusters: a rich toolbox for synthetic biology

Bacteria construct elaborate nanostructures, obtain nutrients and energy from diverse sources, synthesize complex molecules, and implement signal processing to react to their environment. These complex phenotypes require the coordinated action of multiple genes, which are often encoded in a contiguous region of the genome, referred to as a gene cluster. Gene clusters sometimes contain all of the genes necessary and sufficient for a particular function. As an evolutionary mechanism, gene clusters facilitate the horizontal transfer of the complete function between species. Here, we review recent work on a number of clusters whose functions are relevant to biotechnology. Engineering these clusters has been hindered by their regulatory complexity, the need to balance the expression of many genes, and a lack of tools to design and manipulate DNA at this scale. Advances in synthetic biology will enable the large-scale bottom-up engineering of the clusters to optimize their functions, wake up cryptic clusters, or to transfer them between organisms. Understanding and manipulating gene clusters will move towards an era of genome engineering, where multiple functions can be "mixed-and-matched" to create a designer organism.

Analysis of light-induced transmembrane ion gradients and membrane potential in Photosystem I proteoliposomes

Photosystem I (PSI) complexes can support a light-driven electrochemical gradient for protons, which is the driving force for energy-conserving reactions across biological membranes. In this work, a computational model that enables a quantitative description of the light-induced proton gradients across the membrane of PSI proteoliposomes is presented. Using a set of electrodiffusion equations, a compartmental model of a vesicle suspended in aqueous medium was studied. The light-mediated proton movement was modeled as a single proton pumping step with backpressure of the electric potential. The model fits determinations of pH obtained from PSI proteoliposomes illuminated in the presence of mediators of cyclic electron transport. The model also allows analysis of the proton gradients in relation to the transmembrane ion fluxes and electric potential. Sensitivity analysis enabled a determination of the parameters that have greater influence on steady-state levels and onset/decay rates of transmembrane pH and electric potential. This model could be used as a tool for optimizing PSI proteoliposomes for photo-electrochemical applications.