Engineering of supramolecular photoactive protein architectures: the defined co-assembly of photosystem I and cytochrome c using a nanoscaled DNA-matrix

Photosystem I and ZIF-8 Interfacing: Entrapment and Immobilization

In this study, we explore the interfacing of Photosystem I (PSI) with the metal-organic framework (MOF) ZIF-8 (ZIF = zeolitic imidazolate framework) through encapsulation and surface immobilization methods, aimed at stabilizing PSI through biohybrid composite formation. PSI was successfully encapsulated within ZIF-8 (PSI@ZIF-8) and immobilized on ZIF-8 surfaces (PSI/ZIF-8) using a one-pot synthesis and surface impregnation technique, respectively. Characterization techniques including powder X-ray diffraction, Fourier transform infrared spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy confirmed the formation and first-of-its-kind nanoscale visualization of the PSI/ZIF-8 composites. Spectroscopic analysis revealed that while PSI encapsulation resulted in minor structural changes potentially from scaffolding-induced stress and MOF building blocks, the overall protein integrity was maintained. Our study demonstrates that, in contrast to surface interfacing, ZIF-8 encapsulation provides a protective environment for PSI, enhancing its stability and retaining its functional properties, thereby offering an auspicious approach for the development of biohybrid materials in semi-artificial photosynthesis and other biotechnological applications.

Photosystem I complexes form remarkably stable self-assembled tunneling junctions

This paper describes large-area molecular tunneling junctions comprising self-assembled monolayers (SAMs) of light-harvesting protein complexes using eutectic Ga-In (EGaIn) as a top contact. The complexes, which are readily isolable in large quantities from spinach leaves, self-assemble on top of SAMs of [6,6]-phenyl-C61-butyric acid (PCBA) on gold (Au) supported by mica substrates (AuMica), which induces them to adopt a preferred orientation with respect to the electron transport chain that runs across the short axis of each complex, leading to temperature-independent rectification. We compared trimeric protein complexes isolated from thermophilic cyanobacteria to monomeric complexes extracted from spinach leaves by measuring charge-transport at variable temperatures and over the course of at least three months. Transport is independent of temperature in the range of 130 to 310 K for both protein complexes, affirming that the likely mechanism is non-resonant tunneling. The junctions rectified current and were stable for at least three months when stored at room temperature in ambient conditions, with the yield of working junctions falling from 100% to 97% over that time. These results demonstrate a straightforward strategy for forming remarkably robust molecular junctions, avoiding the fragility that is common in molecular electronics.

Dependence of Protein Immobilization and Photocurrent Generation in PSI-FTO Electrodes on the Electrodeposition Parameters

This study investigates the immobilization of cyanobacterial photosystem I (PSI) from Synechocystis sp. PCC 6803 onto fluorine-doped tin oxide (FTO) conducting glass plates to create photoelectrodes for biohybrid solar cells. The fabrication of these PSI-FTO photoelectrodes is based on two immobilization processes: rapid electrodeposition driven by an external electric field and slower adsorption during solvent evaporation, both influenced by gravitational sedimentation. Deposition and performance of photoelectrodes was investigated by UV-Vis absorption spectroscopy and photocurrent measurements. We investigated the efficiency of PSI immobilization under varying conditions, including solution pH, applied electric field intensity and duration, and electrode polarization, with the goals to control (1) the direction of migration and (2) the orientation of the PSI particles on the substrate surface. Variation in the pH value of the PSI solution alters the surface charge distribution, affecting the net charge and the electric dipole moment of these proteins. Results showed PSI migration to the positively charged electrode at pH 6, 7, and 8, and to the negatively charged electrode at pH 4.4 and 5, suggesting an isoelectric point of PSI between 5 and 6. At acidic pH, the electrophoretic migration was largely hindered by protein aggregation. Notably, photocurrent generation was consistently cathodic and correlated with PSI layer thickness, and no conclusions can be drawn on the orientation of the immobilized proteins. Overall, these findings suggest mediated electron transfer from FTO to PSI by the used electrolyte containing 10 mM sodium ascorbate and 200 μM dichlorophenolindophenol.

Photoactive Yellow Protein Adsorption at Hydrated Polyethyleneimine and Poly-l-Glutamic Acid Interfaces

Chiral and achiral vibrational sum-frequency generation (VSFG) spectroscopy was performed in the 1400-1700 and 2800-3800 cm^-1 range to study the interfacial structure of photoactive yellow protein (PYP) adsorbed on polyethyleneimine (PEI) and poly-l-glutamic acid (PGA) surfaces. Nanometer-thick polyelectrolyte layers served as the substrate for PYP adsorption, with 6.5-pair layers providing the most homogeneous surfaces. When the topmost material was PGA, it acquired a random coil structure with a small number of β₂-fibrils. Upon adsorption on oppositely charged surfaces, PYP yielded similar achiral spectra. However, the VSFG signal intensity increased for PGA surfaces with a concomitant redshift of the chiral C^α-H and N-H stretching bands, suggesting increased adsorption for PGA compared to PEI. At low wavenumbers, both the backbone and the side chains of PYP induced drastic changes to all measured chiral and achiral VSFG spectra. Decreasing ambient humidity led to the loss of tertiary structure with a re-orientation of α-helixes, evidenced by a strongly blue-shifted chiral amide I band of the β-sheet structure with a shoulder at 1654 cm^-1. Our observations indicate that chiral VSFG spectroscopy is not only capable of determining the main type of secondary structure of PYP, i.e., β-scaffold, but is also sensitive to tertiary protein structure.

Drop-casted Photosystem I/cytochrome c multilayer films for biohybrid solar energy conversion

One of the main barriers to making efficient Photosystem I-based biohybrid solar cells is the need for an electrochemical pathway to facilitate electron transfer between the P₇₀₀ reaction center of Photosystem I and an electrode. To this end, nature provides inspiration in the form of cytochrome c₆, a natural electron donor to the P₇₀₀ site. Its natural ability to access the P₇₀₀ binding pocket and reduce the reaction center can be mimicked by employing cytochrome c, which has a similar protein structure and redox chemistry while also being compatible with a variety of electrode surfaces. Previous research has incorporated cytochrome c to improve the photocurrent generation of Photosystem I using time consuming and/or specialized electrode preparation. While those methods lead to high protein areal density, in this work we use the quick and facile vacuum-assisted drop-casting technique to construct a Photosystem I/cytochrome c photoactive composite film with micron-scale thickness. We demonstrate that this simple fabrication technique can result in high cytochrome c loading and improvement in cathodic photocurrent over a drop-casted Photosystem I film without cytochrome c. In addition, we analyze the behavior of the cytochrome c/Photosystem I system at varying applied potentials to show that the improvement in performance can be attributed to enhancement of the electron transfer rate to P₇₀₀ sites and therefore the PSI turnover rate within the composite film.

Closing the green gap of photosystem I with synthetic fluorophores for enhanced photocurrent generation in photobiocathodes

One restriction for biohybrid photovoltaics is the limited conversion of green light by most natural photoactive components. The present study aims to fill the green gap of photosystem I (PSI) with covalently linked fluorophores, ATTO 590 and ATTO 532. Photobiocathodes are prepared by combining a 20 μm thick 3D indium tin oxide (ITO) structure with these constructs to enhance the photocurrent density compared to setups based on native PSI. To this end, two electron transfer mechanisms, with and without a mediator, are studied to evaluate differences in the behavior of the constructs. Wavelength-dependent measurements confirm the influence of the additional fluorophores on the photocurrent. The performance is significantly increased for all modifications compared to native PSI when cytochrome c is present as a redox-mediator. The photocurrent almost doubles from -32.5 to up to -60.9 μA cm^-2. For mediator-less photobiocathodes, interestingly, drastic differences appear between the constructs made with various dyes. While the turnover frequency (TOF) is doubled to 10 e^-/PSI/s for PSI-ATTO590 on the 3D ITO compared to the reference specimen, the photocurrents are slightly smaller since the PSI-ATTO590 coverage is low. In contrast, the PSI-ATTO532 construct performs exceptionally well. The TOF increases to 31 e^-/PSI/s, and a photocurrent of -47.0 μA cm^-2 is obtained. This current is a factor of 6 better than the reference made with native PSI in direct electron transfer mode and sets a new record for mediator-free photobioelectrodes combining 3D electrode structures and light-converting biocomponents.

Observation of anomalous carotenoid and blind chlorophyll activations in photosystem I under synthetic membrane confinements

The role of natural thylakoid membrane confinements in architecting the robust structural and electrochemical properties of PSI is not fully understood. Most PSI studies till date extract the proteins from their natural confinements that can lead to non-native conformations. Recently our group had successfully reconstituted PSI in synthetic lipid membranes using detergent-mediated liposome solubilizations. In this study, we investigate the alterations in chlorophylls and carotenoids interactions and reorganization in PSI based on spectral property changes induced by its confinement in anionic DPhPG and zwitterionic DPhPC phospholipid membranes. To this end, we employ a combination of absorption, fluorescence, and circular dichroism (CD) spectroscopic measurements. Our results indicate unique activation and alteration of photoresponses from the PSI carotenoid (Car) bands in PSI-DPhPG proteoliposomes that can tune the Excitation Energy Transfer (EET), otherwise absent in PSI at non-native environments. Specifically, we observe broadband light harvesting via enhanced absorption in the otherwise non-absorptive green region (500-580 nm) of the Chlorophylls (Chl) along with ~64% increase in the full-width half maximum of the Qy band (650-720 nm). The CD results indicate enhanced Chl-Chl and Chl-Car interactions along with conformational changes in protein secondary structures. Such distinct changes in the Car and Chl bands are not observed in PSI confined in DPhPC. The fundamental insights into membrane microenvironments tailoring PSI subunits reorganization and interactions provide novel strategies for tuning photoexcitation processes and rational designing of biotic-abiotic interfaces in PSI-based photoelectrochemical energy conversion systems.

Electron Transfer in a Bio-Photoelectrode Based on Photosystem I Multilayer Immobilized on the Conducting Glass

A film of ~40 layers of partially oriented photosystem I (PSI) complexes isolated from the red alga Cyanidioschyzon merolae formed on the conducting glass through electrodeposition was investigated by time-resolved absorption spectroscopy and chronoamperometry. The experiments were performed at a range of electric potentials applied to the film and at different compositions of electrolyte solution being in contact with the film. The amount of immobilized proteins supporting light-induced charge separation (active PSI) ranged from ~10%, in the absence of any reducing agents (redox compounds or low potential), to ~20% when ascorbate and 2,6-dichlorophenolindophenol were added, and to ~35% when the high negative potential was additionally applied. The origin of the large fraction of permanently inactive PSI (65–90%) was unclear. Both reducing agents increased the subpopulation of active PSI complexes, with the neutral P700 primary electron donor, by reducing significant fractions of the photo-oxidized P700 species. The efficiencies of light-induced charge separation in the PSI film (10–35%) did not translate into an equally effective generation of photocurrent, whose internal quantum efficiency reached the maximal value of 0.47% at the lowest potentials. This mismatch indicates that the vast majority of the charge-separated states in multilayered PSI complexes underwent charge recombination.

"Invisible" Detergents Enable a Reliable Determination of Solution Structures of Native Photosystems by Small-Angle Neutron Scattering

Photosystems I (PSI) and II (PSII) are pigment-protein complexes capable of performing the light-induced charge separation necessary to convert solar energy into a biochemically storable form, an essential step in photosynthesis. Small-angle neutron scattering (SANS) is unique in providing structural information on PSI and PSII in solution under nearly physiological conditions without the need for crystallization or temperature decrease. We show that the reliability of the solution structure critically depends on proper contrast matching of the detergent belt surrounding the protein. Especially, specifically deuterated ("invisible") detergents are shown to be properly matched out in SANS experiments by a direct, quantitative comparison with conventional matching strategies. In contrast, protonated detergents necessarily exhibit incomplete matching so that related SANS results systematically overestimate the size of the membrane protein under study. While the solution structures obtained are close to corresponding high-resolution structures, we show that temperature and solution state lead to individual structural differences compared with high-resolution structures. We attribute these differences to the presence of a manifold of conformational substates accessible by protein dynamics under physiological conditions.

Graphene oxide decorated with gold enables efficient biophotovolatic cells incorporating photosystem I

This paper describes the use of reduced graphene oxide decorated with gold nanoparticles as an efficient electron transfer layer for solid-state biophotovoltic cells containing photosystem I as the sole photo-active component. Together with polytyrosine–polyaniline as a hole transfer layer, this device architecture results in an open-circuit voltage of 0.3 V, a fill factor of 38% and a short-circuit current density of 5.6 mA cm⁻² demonstrating good coupling between photosystem I and the electrodes. The best-performing device reached an external power conversion efficiency of 0.64%, the highest for any solid-state photosystem I-based photovoltaic device that has been reported to date. Our results demonstrate that the functionality of photosystem I in the non-natural environment of solid-state biophotovoltaic cells can be improved through the modification of electrodes with efficient charge-transfer layers. The combination of reduced graphene oxide with gold nanoparticles caused tailoring of the electronic structure and alignment of the energy levels while also increasing electrical conductivity. The decoration of graphene electrodes with gold nanoparticles is a generalizable approach for enhancing charge-transfer across interfaces, particularly when adjusting the levels of the active layer is not feasible, as is the case for photosystem I and other biological molecules.

This paper describes the use of reduced graphene oxide decorated with gold nanoparticles as an efficient electron transport layer for solid-state biophotovoltic cells containing photosystem I as the sole photo-active component.

Competition between intra-protein charge recombination and electron transfer outside photosystem I complexes used for photovoltaic applications

Photosystem I (PSI) complexes isolated from three different species were electrodeposited on FTO conducting glass, forming a photoactive multilayer of the photo-electrode, for investigation of intricate electron transfer (ET) properties in such green hybrid nanosystems. The internal quantum efficiency of photo-electrochemical cells (PEC) containing the PSI-based photo-electrodes did not exceed ~ 0.5%. To reveal the reason for such a low efficiency of photocurrent generation, the temporal evolution of the transient concentration of the photo-oxidized primary electron donor, P⁺, was studied in aqueous suspensions of the PSI complexes by time-resolved absorption spectroscopy. The results of these measurements provided the information on: (1) completeness of charge separation in PSI reaction centers (RCs), (2) dynamics of internal charge recombination, and (3) efficiency of electron transfer from PSI to the electrolyte, which is the reaction competing with the internal charge recombination in the PSI RC. The efficiency of the full charge separation in the PSI complexes used for functionalization of the electrodes was ~ 90%, indicating that incomplete charge separation was not the main reason for the small yield of photocurrents. For the PSI particles isolated from a green alga Chlamydomonas reinhardtii, the probability of ET outside PSI was ~ 30-40%, whereas for their counterparts isolated from a cyanobacterium Synechocystis sp. PCC 6803 and a red alga Cyanidioschyzon merolae, it represented a mere ~ 4%. We conclude from the transient absorption data for the PSI biocatalysts in solution that the observed small photocurrent efficiency of ~ 0.5% for all the PECs analyzed in this study is likely due to: (1) limited efficiency of ET outside PSI, particularly in the case of PECs based on PSI from Synechocystis and C. merolae, and (2) the electrolyte-mediated electric short-circuiting in PSI particles forming the photoactive layer, particularly in the case of the C. reinhardtii PEC.

Electrospinning for building 3D structured photoactive biohybrid electrodes

We describe the development of biohybrid electrodes constructed via combination of electrospun (e-spun) 3D indium tin oxide (ITO) with the trimeric supercomplex photosystem I and the small electrochemically active protein cytochrome c (cyt c). The developed 3D surface of ITO has been created by electrospinning of a mixture of polyelthylene oxide (PEO) and ITO nanoparticles onto ITO glass slides followed by a subsequent elimination of PEO by sintering the composite. Whereas the photosystem I alone shows only small photocurrents at these 3D electrodes, the co-immobilization of cyt c to the e-spun 3D ITO results in well-defined photoelectrochemical signals. The scaling of thickness of the 3D ITO layers by controlling the time (10 min and 60 min) of electrospinning results in enhancement of the photocurrent. Several performance parameters of the electrode have been analyzed for different illumination intensities.

Layer-by-Layer Assembly of Photosystem I and PEDOT:PSS Biohybrid Films for Photocurrent Generation

The design of electrode interfaces to achieve efficient electron transfer to biomolecules is important in many bioelectrochemical processes. Within the field of biohybrid solar energy conversion, constructing multilayered Photosystem I (PSI) protein films that maintain good electronic connection to an underlying electrode has been a major challenge. Previous shortcomings include low loadings, long deposition times, and poor connection between PSI and conducting polymers within composite films. Here, we show that PSI protein complexes can be deposited into monolayers within a 30 min timespan by leveraging the electrostatic interactions between the protein complex and the poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) polymer. Further, we follow a layer-by-layer (LBL) deposition procedure to produce up to 9-layer pairs of PSI and PEDOT:PSS with highly reproducible layer thicknesses as well as distinct changes in surface composition. When tested in an electrochemical cell employing ubiquinone-0 as a mediator, the photocurrent performance of the LBL films increased linearly by 83 ± 6 nA/cm² per PSI layer up to 6-layer pairs. The 6-layer pair samples yielded a photocurrent of 414 ± 13 nA/cm², after which the achieved photocurrent diminished with additional layer pairs. The turnover number (TN) of the PSI-PEDOT:PSS LBL assemblies also greatly exceeds that of drop-casted PSI multilayer films, highlighting that the rate of electron collection is improved through the systematic deposition of the protein complexes and conducting polymer. The capability to deposit high loadings of PSI between PEDOT:PSS layers, while retaining connection to the underlying electrode, shows the value in using LBL assembly to produce PSI and PEDOT:PSS bioelectrodes for photoelectrochemical energy harvesting applications.

Photosystem I integrated into mesoporous microspheres has enhanced stability and photoactivity in biohybrid solar cells

Isolated proteins, especially membrane proteins, are susceptible to aggregation and activity loss after purification. For therapeutics and biosensors usage, protein stability and longevity are especially important. It has been demonstrated that photosystem I (PSI) can be successfully integrated into biohybrid electronic devices to take advantage of its strong light-driven reducing potential (-1.2V vs. the Standard Hydrogen Electrode). Most devices utilize PSI isolated in a nanosize detergent micelle, which is difficult to visualize, quantitate, and manipulate. Isolated PSI is also susceptible to aggregation and/or loss of activity, especially after freeze/thaw cycles. CaCO3 microspheres (CCMs) have been shown to be a robust method of protein encapsulation for industrial and pharmaceutical applications, increasing the stability and activity of the encapsulated protein. However, CCMs have not been utilized with any membrane protein(s) to date. Herein, we examine the encapsulation of detergent-solubilized PSI in CCMs yielding uniform, monodisperse, mesoporous microspheres. This study reports both the first encapsulation of a membrane protein and also the largest protein to date stabilized by CCMs. These microspheres retain their spectral properties and lumenal surface exposure and are active when integrated into hybrid biophotovoltaic devices. CCMs may be a robust yet simple solution for long-term storage of large membrane proteins, showing success for very large, multisubunit complexes like PSI.

Scalable Three-Dimensional Photobioelectrodes Made of Reduced Graphene Oxide Combined with Photosystem I

Photobioelectrodes represent one of the examples where artificial materials are combined with biological entities to undertake semi-artificial photosynthesis. Here, an approach is described that uses reduced graphene oxide (rGO) as an electrode material. This classical 2D material is used to construct a three-dimensional structure by a template-based approach combined with a simple spin-coating process during preparation. Inspired by this novel material and photosystem I (PSI), a biophotovoltaic electrode is being designed and investigated. Both direct electron transfer to PSI and mediated electron transfer via cytochrome c from horse heart as redox protein can be confirmed. Electrode preparation and protein immobilization have been optimized. The performance can be upscaled by adjusting the thickness of the 3D electrode using different numbers of spin-coating steps during preparation. Thus, photocurrents up to ∼14 μA/cm² are measured for 12 spin-coated layers of rGO corresponding to a turnover frequency of 30 e^- PSI^-1 s^-1 and external quantum efficiency (EQE) of 0.07% at a thickness of about 15 μm. Operational stability has been analyzed for several days. Particularly, the performance at low illumination intensities is very promising (1.39 μA/cm² at 0.1 mW/cm² and -0.15 V vs Ag/AgCl; EQE 6.8%).

Membrane Protein Modified Electrodes in Bioelectrocatalysis

Transmembrane proteins involved in metabolic redox reactions and photosynthesis catalyse a plethora of key energy-conversion processes and are thus of great interest for bioelectrocatalysis-based applications. The development of membrane protein modified electrodes has made it possible to efficiently exchange electrons between proteins and electrodes, allowing mechanistic studies and potentially applications in biofuels generation and energy conversion. Here, we summarise the most common electrode modification and their characterisation techniques for membrane proteins involved in biofuels conversion and semi-artificial photosynthesis. We discuss the challenges of applications of membrane protein modified electrodes for bioelectrocatalysis and comment on emerging methods and future directions, including recent advances in membrane protein reconstitution strategies and the development of microbial electrosynthesis and whole-cell semi-artificial photosynthesis.

A new platform for development of photosystem I based thin films with superior photocurrent: TCNQ charge transfer salts derived from ZIF-8

The transmembrane photosynthetic protein complex Photosystem I (PSI) is highly sought after for incorporation into biohybrid photovoltaic devices due to its remarkable photoactive electrochemical properties, chiefly driving charge separation with ∼1 V potential and ∼100% quantum efficiency. In pursuit of these integrated technologies, three factors must be simultaneously tuned, namely, direct redox transfer steps, three-dimensional coordination and stabilization of PSI aggregates, and interfacial connectivity with conductive pathways. Building on our recent successful encapsulation of PSI in the metal-organic framework ZIF-8, herein we use the zinc and imidazole cations from this precursor to form charge transfer complexes with an extremely strong organic electron acceptor, TCNQ. Specifically, the PSI-Zn-H2mim-TCNQ charge transfer salt complex was drop cast on ITO to form dense films. Subsequent voltammetric cycling induced cation exchange and electrochemical annealing of the film was used to enhance electron conductivity giving rise to a photocurrent in the order of 15 μA cm-2. This study paves the way for a myriad of future opportunities for successful integration of this unique class of charge transfer salt complexes with biological catalysts and light harvesters.

Putting Photosystem I to Work: Truly Green Energy

Meeting growing energy demands sustainably is one of the greatest challenges facing the world. The sun strikes the Earth with sufficient energy in 1.5 h to meet annual world energy demands, likely making solar energy conversion part of future sustainable energy production plans. Photosynthetic organisms have been evolving solar energy utilization strategies for nearly 3.5 billion years, making reaction centers including the remarkably stable Photosystem I (PSI) especially interesting for biophotovoltaic device integration. Although these biohybrid devices have steadily improved, their output remains low compared with traditional photovoltaics. We discuss strategies and methods to improve PSI-based biophotovoltaics, focusing on PSI-surface interaction enhancement, electrolytes, and light-harvesting enhancement capabilities. Desirable features and current drawbacks to PSI-based devices are also discussed.

A Robust Bioderived Wavelength-Specific Photosensor Based on BLUF Proteins

Photosensitive proteins are naturally evolved photosensors that often respond to light signals of specific wavelengths. However, their poor stability under ambient conditions hinders their applications in non-biological settings. In this proof-of-principle study, we grafted the blue light using flavin (BLUF) protein reconstructed with flavin adenine dinucleotide (FAD) or roseoflavin (RoF) onto pristine graphene, and achieved selective sensitivity at 450 nm or 500 nm, respectively. We improved the thermal and operational stability substantially via structure-guided cross-linking, achieving 6-month stability under ambient condition and normal operation at temperatures up to 200 °C. Furthermore, the device exhibited rare negative photoconductivity behavior. The origins of this negative photoconductivity behavior were elucidated via a combination of experimental and theoretical analysis. In the photoelectric conversion studies, holes from photoexcited flavin migrated to graphene and recombined with electrons. The device allows facile modulation and detection of charge transfer, and provides a versatile platform for future studies of photoinduced charge transfer in biosensors as well as the development of stable wavelength-selective biophotosensors.

Excitation dynamics in Photosystem I trapped in TiO2 mesopores

Excitation decay in closed Photosystem I (PSI) isolated from cyanobacterium Synechocystis sp. PCC 6803 and dissolved in a buffer solution occurs predominantly with a ~ 24-ps lifetime, as measured both by time-resolved fluorescence and transient absorption. The same PSI particles deposited in mesoporous matrix made of TiO₂ nanoparticles exhibit significantly accelerated excitation decay dominated by a ~ 6-ps component. Target analysis indicates that this acceleration is caused by ~ 50% increase of the rate constant of bulk Chls excitation quenching. As an effect of this increase, as much as ~ 70% of bulk Chls excitation is quenched before the establishment of equilibrium with the red Chls. Accelerated quenching may be caused by increased excitation trapping by the reaction center and/or quenching properties of the TiO₂ surface directly interacting with PSI Chls. Also properties of the PSI red Chls are affected by the deposition in the TiO₂ matrix: they become deeper traps due to an increase of their number and their oscillator strength is significantly reduced. These effects should be taken into account when constructing solar cells' photoelectrodes composed of PSI and artificial matrices.

Photovoltaic activity of electrodes based on intact photosystem I electrodeposited on bare conducting glass

We demonstrate photovoltaic activity of electrodes composed of fluorine-doped tin oxide (FTO) conducting glass and a multilayer of trimeric photosystem I (PSI) from cyanobacterium Synechocystis sp. PCC 6803 yielding, at open circuit potential (OCP) of + 100 mV (vs. SHE), internal quantum efficiency of (0.37 ± 0.11)% and photocurrent density of up to (0.5 ± 0.1) µA/cm². The photocurrent measured for OCP is of cathodic nature meaning that preferentially the electrons are injected from the conducting layer of the FTO glass to the photooxidized PSI primary electron donor, P700⁺, and further transferred from the photoreduced final electron acceptor of PSI, F_b^-, via ascorbate electrolyte to the counter electrode. This observation is consistent with preferential donor-side orientation of PSI on FTO imposed by applied electrodeposition. However, by applying high-positive bias (+ 620 mV) to the PSI-FTO electrode, exceeding redox midpoint potential of P700 (+ 450 mV), the photocurrent reverses its orientation and becomes anodic. This is explained by "switching off" the natural photoactivity of PSI particles (by the electrochemical oxidation of P700 to P700⁺) and "switching on" the anodic photocurrent from PSI antenna Chls prone to photooxidation at high potentials. The efficient control of the P700 redox state (P700 or P700⁺) by external bias applied to the PSI-FTO electrodes was evidenced by ultrafast transient absorption spectroscopy. The advantage of the presented system is its structural simplicity together with in situ-proven high intactness of the PSI particles.

A precursor-approach in constructing 3D ITO electrodes for the improved performance of photosystem I-cyt c photobioelectrodes

In recent years the use of photoelectrodes based on conductive metal oxides has become very popular in the field of photovoltaics. The application of 3D electrodes holds great promise since they can integrate large amounts of photoactive proteins. In this study photosystem I (PSI) from the thermophilic cyanobacterium Thermosynechococcus elongatus was immobilized on 3D ITO electrodes and electrically wired via the redox protein cytochrome c (cyt c). The main goal, however, was the investigation of construction parameters of such electrodes for achieving a high performance. For this, ITO electrodes were constructed from liquid precursors resulting in improved transmission compared to previous nanoparticle-based preparation protocols. First, the doping level of Sn was varied for establishing suitable conditions for a fast cyt c electrochemistry on such 3D electrodes. In a second step the pore diameter was varied in order to elucidate optimal conditions. Third, the scalability of the template-based preparation was studied from 3 to 15 layers during spin coating and the subsequent baking step. In the thickness range from 3 to 17 μm no limitation in the protein immobilization and also in the photocurrent generation was found. Consequently, a photocurrent of about 270 μA cm-2 and a turnover number (Te) of 30 e- s-1 at PSI were achieved. Because of the high current flow the withdrawal of electrons at the stromal side of PSI becomes clearly rate limiting. Here improved transport conditions and alternative electron acceptors were studied to overcome this limitation.

The Future of Layer-by-Layer Assembly: A Tribute to ACS Nano Associate Editor Helmuth Möhwald

Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings. In this Perspective, dedicated to the memory of ACS Nano associate editor Prof. Dr. Helmuth Möhwald, we discuss the developments and applications that are to come in LbL assembly, focusing on coatings, bulk materials, membranes, nanocomposites, and delivery vehicles.

Extended Operational Lifetime of a Photosystem-Based Bioelectrode

The development of bioelectrochemical assemblies for sustainable energy transformation constitutes an increasingly important field of research. Significant progress has been made in the development of semiartificial devices for conversion of light into electrical energy by integration of photosynthetic biomolecules on electrodes. However, sufficient long-term stability of such biophotoelectrodes has been compromised by reactive species generated under aerobic operation. Therefore, meeting the requirements of practical applications still remains unsolved. We present the operation of a photosystem I-based photocathode using an electron acceptor that enables photocurrent generation under anaerobic conditions as the basis for a biodevice with substantially improved stability. A continuous operation lifetime considerably superior to previous reports and at higher light intensities is paving the way toward the potential application of semiartificial energy conversion devices.

Jolly green MOF: confinement and photoactivation of photosystem I in a metal-organic framework

Photosystem I (PSI) is a ∼1000 kDa transmembrane protein that enables photoactivated charge separation with ∼1 V driving potential and ∼100% quantum efficiency during the photosynthetic process. Although such properties make PSI a potential candidate for integration into bio-hybrid solar energy harvesting devices, the grand challenge in orchestrating such integration rests on rationally designed 3D architectures that can organize and stabilize PSI in the myriad of harsh conditions in which it needs to function. The current study investigates the optical response and photoactive properties of PSI encapsulated in a highly stable nanoporous metal-organic framework (ZIF-8), denoted here as PSI@ZIF-8. The ZIF-8 framework provides a unique scaffold with a robust confining environment for PSI while protecting its precisely coordinated chlorophyll networks from denaturing agents. Significant blue shifts in the fluorescence emissions from UV-vis measurements reveal the successful confinement of PSI in ZIF-8. Pump-probe spectroscopy confirms the photoactivity of the PSI@ZIF-8 composites by revealing the successful internal charge separation and external charge transfer of P₇₀₀ ⁺ and F_B ^- even after exposure to denaturing agents and organic solvents. This work provides greater fundamental understanding of confinement effects on pigment networks, while significantly broadening the potential working environments for PSI-integrated bio-hybrid materials.

Light-induced formation of partially reduced oxygen species limits the lifetime of photosystem 1-based biocathodes

Interfacing photosynthetic proteins specifically photosystem 1 (PS1) with electrodes enables light-induced charge separation processes for powering semiartificial photobiodevices with, however, limited long-term stability. Here, we present the in-depth evaluation of a PS1/Os-complex-modified redox polymer-based biocathode by means of scanning photoelectrochemical microscopy. Focalized local illumination of the bioelectrode and concomitant collection of H₂O₂ at the closely positioned microelectrode provide evidence for the formation of partially reduced oxygen species under light conditions. Long-term evaluation of the photocathode at different O₂ concentrations as well as after incorporating catalase and superoxide dismutase reveals the particularly challenging issue of avoiding the generation of reactive species. Moreover, the evaluation of films prepared with inactivated PS1 and free chlorophyll points out additional possible pathways for the generation of oxygen radicals. To avoid degradation of PS1 during illumination and hence to enhance the long-term stability, the operation of biophotocathodes under anaerobic conditions is indispensable.

Insights into the binding behavior of native and non-native cytochromes to photosystem I from Thermosynechococcus elongatus

The binding of photosystem I (PS I) from Thermosynechococcus elongatus to the native cytochrome (cyt) c₆ and cyt c from horse heart (cyt c_HH) was analyzed by oxygen consumption measurements, isothermal titration calorimetry (ITC), and rigid body docking combined with electrostatic computations of binding energies. Although PS I has a higher affinity for cyt c_HH than for cyt c₆, the influence of ionic strength and pH on binding is different in the two cases. ITC and theoretical computations revealed the existence of unspecific binding sites for cyt c_HH besides one specific binding site close to P₇₀₀ Binding to PS I was found to be the same for reduced and oxidized cyt c_HH Based on this information, suitable conditions for cocrystallization of cyt c_HH with PS I were found, resulting in crystals with a PS I:cyt c_HH ratio of 1:1. A crystal structure at 3.4-Å resolution was obtained, but cyt c_HH cannot be identified in the electron density map because of unspecific binding sites and/or high flexibility at the specific binding site. Modeling the binding of cyt c₆ to PS I revealed a specific binding site where the distance and orientation of cyt c₆ relative to P₇₀₀ are comparable with cyt c₂ from purple bacteria relative to P₈₇₀ This work provides new insights into the binding modes of different cytochromes to PS I, thus facilitating steps toward solving the PS I-cyt c costructure and a more detailed understanding of natural electron transport processes.

Enhancing Photocurrent Generation in Photosynthetic Reaction Center-Based Photoelectrochemical Cells with Biomimetic DNA Antenna

Three- to four-times higher performance of biohybrid photoelectrochemical cells with photosynthetic reaction centers (RC) has been achieved by using a DNA-based biomimetic antenna. Synthetic dyes Cy3 and Cy5 were chosen and strategically placed in the anntena in such a way that they can collect additional light energy in the visible region of the solar spectrum and transfer it to RC through Förster resonance energy transfer (FRET). The antenna, a DNA templated multiple dye system, is attached to each Rhodobacter sphaeroides RC near the primary donor, P, to facilitate the energy transfer process. Excitation with a broad light spectrum (approximating sunlight) triggers a cascade of excitation energy transfer from Cy3 to Cy5 to P, and also directly from Cy5 to P. This additional excitation energy increases the RC absorbance cross-section in the visible and thus the performance of the photoelectrochemical cells. DNA-based biomimetic antennas offer a tunable, modular light-harvesting system for enhancing RC solar coverage and performance for photoelectrochemical cells.

Bioelectronic Circuit on a 3D Electrode Architecture: Enzymatic Catalysis Interconnected with Photosystem I

Artificial light-driven signal chains are particularly important for the development of systems converting light into a current, into chemicals or for light-induced sensing. Here, we report on the construction of an all-protein, light-triggered, catalytic circuit based on photosystem I, cytochrome c (cyt c) and human sulfite oxidase (hSOX). The defined assembly of all components using a modular design results in an artificial biohybrid electrode architecture, combining the photophysical features of PSI with the biocatalytic properties of hSOX for advanced light-controlled bioelectronics. The working principle is based on a competitive switch between electron supply from the electrode or by enzymatic substrate conversion.

Solution structure of monomeric and trimeric photosystem I of Thermosynechococcus elongatus investigated by small-angle X-ray scattering

The structure of monomeric and trimeric photosystem I (PS I) of Thermosynechococcus elongatus BP1 (T. elongatus) was investigated by small-angle X-ray scattering (SAXS). The scattering data reveal that the protein-detergent complexes possess radii of gyration of 58 and 78 Å in the cases of monomeric and trimeric PS I, respectively. The results also show that the samples are monodisperse, virtually free of aggregation, and contain empty detergent micelles. The shape of the protein-detergent complexes can be well approximated by elliptical cylinders with a height of 78 Å. Monomeric PS I in buffer solution exhibits minor and major radii of the elliptical cylinder of about 50 and 85 Å, respectively. In the case of trimeric PS I, both radii are equal to about 110 Å. The latter model can be shown to accommodate three elliptical cylinders equal to those describing monomeric PS I. A structure reconstitution also reveals that the protein-detergent complexes are larger than their respective crystal structures. The reconstituted structures are larger by about 20 Å mainly in the region of the hydrophobic surfaces of the monomeric and trimeric PS I complexes. This seeming contradiction can be resolved by the addition of a detergent belt constituted by a monolayer of dodecyl-β-D-maltoside molecules. Assuming a closest possible packing, a number of roughly 1024 and 1472 detergent molecules can be determined for monomeric and trimeric PS I, respectively. Taking the monolayer of detergent molecules into account, the solution structure can be almost perfectly modeled by the crystal structures of monomeric and trimeric PS I.

Tuning the photoexcitation response of cyanobacterial Photosystem I via reconstitution into Proteoliposomes

The role of natural thylakoid membrane housing of Photosystem I (PSI), the transmembrane photosynthetic protein, in its robust photoactivated charge separation with near unity quantum efficiency is not fundamentally understood. To this end, incorporation of suitable protein scaffolds for PSI incorporation is of great scientific and device manufacturing interest. Areas of interest include solid state bioelectronics, and photoelectrochemical devices that require bio-abio interfaces that do not compromise the photoactivity and photostability of PSI. Therefore, the surfactant-induced membrane solubilization of a negatively charged phospholipid (DPhPG) with the motivation of creating biomimetic reconstructs of PSI reconstitution in DPhPG liposomes is studied. Specifically, a simple yet elegant method for incorporation of PSI trimeric complexes into DPhPG bilayer membranes that mimic the natural thylakoid membrane housing of PSI is introduced. The efficacy of this method is demonstrated via absorption and fluorescence spectroscopy measurements as well as direct visualization using atomic force microscopy. This study provides direct evidence that PSI confinements in synthetic lipid scaffolds can be used for tuning the photoexcitation characteristics of PSI. Hence, it paves the way for development of fundamental understanding of microenvironment alterations on photochemical response of light activated membrane proteins.

Photocatalytic photosystem I/PEDOT composite films prepared by vapor-phase polymerization

Photosystem I (PSI) achieves photo-induced charge separation with outstanding internal quantum efficiency and has been used to improve the performance of various photoelectrochemical systems. Herein, we describe a fast and versatile technique to assemble composite films containing PSI and a chosen intrinsically conductive polymer (ICP). A mixture of PSI and a Friedel-Crafts catalyst (FeCl₃) is drop cast atop a substrate of choice. Contact with ICP monomer vapor at low temperature stimulates polymer growth throughout PSI films in minutes. We assess the effects of PSI loading on the rapid vapor-phase growth of poly(3,4-ethylenedioxythiophene) (PEDOT) within and above PSI multilayer films, and characterize the resulting film's thickness, electrochemical capacitance, and photocatalytic response. Composite films generate cathodic photocurrent when in contact with an aqueous redox electrolyte, confirming retention of the photocatalytic activity of the polymer-entrapped PSI multilayer assembly.

Supramolecular electrode assemblies for bioelectrochemistry

For more than three decades, the field of bioelectrochemistry has provided novel insights into the catalytic mechanisms of enzymes, the principles that govern biological electron transfer, and has elucidated the basic principles for bioelectrocatalytic systems. Progress in biochemistry, bionanotechnology, and our ever increasing ability to control the chemistry and structure of electrode surfaces has enabled the study of ever more complex systems with bioelectrochemistry. This feature article highlights developments over the last decade, where supramolecular approaches have been employed to develop electrode assemblies that increase enzyme loading on the electrode or create more biocompatible environments for membrane enzymes. Two approaches are particularly highlighted: the use of layer-by-layer assembly, and the modification of electrodes with planar lipid membranes.

Insights into Interprotein Electron Transfer of Human Cytochrome c Variants Arranged in Multilayer Architectures by Means of an Artificial Silica Nanoparticle Matrix

The redox behavior of proteins plays a crucial part in the design of bioelectronic systems. We have demonstrated several functional systems exploiting the electron exchange properties of the redox protein cytochrome c (cyt c) in combination with enzymes and photoactive proteins. The operation is based on an effective reaction at modified electrodes but also to a large extent on the capability of self-exchange between cyt c molecules in a surface-fixed state. In this context, different variants of human cyt c have been examined here with respect to an altered heterogeneous electron transfer (ET) rate in a monolayer on electrodes as well as an enhanced self-exchange rate while being incorporated in multilayer architectures. For this purpose, mutants of the wild-type (WT) protein have been prepared to change the chemical nature of the surface contact area near the heme edge. The structural integrity of the variants has been verified by NMR and UV-vis measurements. It is shown that the single-point mutations can significantly influence the heterogeneous ET rate at thiol-modified gold electrodes and that electroactive protein/silica nanoparticle multilayers can be constructed with all forms of human cyt c prepared. The kinetic behavior of electron exchange for the mutant proteins in comparison with that of the WT has been found altered in some multilayer arrangements. Higher self-exchange rates have been found for K79A. The results demonstrate that the position of the introduced change in the charge situation of cyt c has a profound influence on the exchange behavior. In addition, the behavior of the cyt c variants in assembled multilayers is found to be rather similar to the situation of cyt c self-exchange in solution verified by NMR.