Visualization of Molecular Permeation into a Multi-compartment Phospholipid Vesicle.

Passive permeation of small molecules into vesicles with multiple compartments is a critical event in many chemical and biological processes. We consider the translocation of the peptide NAF-144-67 labeled with a fluorescent fluorescein dye across membranes of rhodamine-labeled 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) into liposomes with internal vesicles. Time-resolved microscopy revealed a sequential absorbance of the peptide in both the outer and inner micrometer vesicles that developed over a time period of minutes to hours, illustrating the spatial and temporal progress of the permeation. There is minimal perturbation of the membrane structure and no evidence for pore formation. On the basis of molecular dynamics simulations of NAF-144-67, we extended a local defect model to migration processes that include multiple compartments. The model captures the long residence time of the peptide within the membrane and the rate of permeation through the liposome and its internal compartments. Imaging experiments confirm the semi-quantitative description of the permeation of the model by activated diffusion and open the way for studies of more complex systems.

Cryo-EM photosystem I structure reveals adaptation mechanisms to extreme high light in Chlorella ohadii.

Photosynthesis in deserts is challenging since it requires fast adaptation to rapid night-to-day changes, that is, from dawn's low light (LL) to extreme high light (HL) intensities during the daytime. To understand these adaptation mechanisms, we purified photosystem I (PSI) from Chlorella ohadii, a green alga that was isolated from a desert soil crust, and identified the essential functional and structural changes that enable the photosystem to perform photosynthesis under extreme high light conditions. The cryo-electron microscopy structures of PSI from cells grown under low light (PSILL) and high light (PSIHL), obtained at 2.70 and 2.71 Å, respectively, show that part of light-harvesting antenna complex I (LHCI) and the core complex subunit (PsaO) are eliminated from PSIHL to minimize the photodamage. An additional change is in the pigment composition and their number in LHCIHL; about 50% of chlorophyll b is replaced by chlorophyll a. This leads to higher electron transfer rates in PSIHL and might enable C. ohadii PSI to act as a natural photosynthesiser in photobiocatalytic systems. PSIHL or PSILL were attached to an electrode and their induced photocurrent was determined. To obtain photocurrents comparable with PSIHL, 25 times the amount of PSILL was required, demonstrating the high efficiency of PSIHL. Hence, we suggest that C. ohadii PSIHL is an ideal candidate for the design of desert artificial photobiocatalytic systems.

Gated Dissipative Dynamic Artificial Photosynthetic Model Systems.

Gated dissipative artificial photosynthetic systems modeling dynamically modulated environmental effects on the photosynthetic apparatus are presented. Two photochemical systems composed of a supramolecular duplex scaffold, a photosensitizer-functionalized strand (photosensitizer is Zn(II)protoporphyrin IX, Zn(II)PPIX, or pyrene), an electron acceptor bipyridinium (V2+)-modified strand, and a nicking enzyme (Nt.BbvCI) act as functional assemblies driving transient photosynthetic-like processes. In the presence of a fuel strand, the transient electron transfer quenching of the photosensitizers, in each of the photochemical systems, is activated. In the presence of a sacrificial electron donor (mercaptoethanol) and continuous irradiation, the resulting electron transfer process in the Zn(II)PPIX/V2+ photochemical module leads to the transient accumulation and depletion of the bipyridinium radical-cation (V·+) product, and in the presence of ferredoxin-NADP+ reductase and NADP+, to the kinetically modulated photosynthesis of NADPH. By subjecting the mixture of two photochemical modules to one of two inhibitors, the gated transient photoinduced electron transfer in the two modules is demonstrated. Such gated dissipative process highlights its potential as an important pathway to protect artificial photosynthetic module against overdose of irradiance and to minimize photodamage.

Integration of photocatalytic and dark-operating catalytic biomimetic transformations through DNA-based constitutional dynamic networks.

Nucleic acid-based constitutional dynamic networks (CDNs) have recently emerged as versatile tools to control a variety of catalytic processes. A key challenge in the application of these systems is achieving intercommunication between different CDNs to mimic the complex interlinked networks found in cellular biology. In particular, the possibility to interface photochemical 'energy-harvesting' processes with dark-operating 'metabolic' processes, in a similar way to plants, represents an up to now unexplored yet enticing research direction. The present study introduces two CDNs that allow the intercommunication of photocatalytic and dark-operating catalytic functions mediated by environmental components that facilitate the dynamic coupling of the networks. The dynamic feedback-driven intercommunication of the networks is accomplished via information transfer between the two CDNs effected by hairpin fuel strands in the environment of the system, leading to the coupling of the photochemical and dark-operating modules.

Photosynthesis Z-Scheme biomimicry: Photosystem I/BiVO4 photo-bioelectrochemical cell for donor-free bias-free electrical power generation.

Photo-bioelectrochemical cells that are based on photosynthetic proteins are drawing increased attention for both fundamental and applied research. While novel photosynthetic based systems have been introduced, further optimization in terms of stability and efficiency is required. Photosystem I has been utilized extensively in bioelectronic devices, often in conjugation with viologen moieties which act as electron acceptors. It has been shown previously that a partial reduction of oxygen to H2O2 can facilitate damage to proteins hence, limits their long-term activation. Here, we show a newly developed bias-free, donor-free photo-bioelectrochemical system that mimics the natural photosynthetic Z-scheme. Polymethylene blue and polybutyl-viologen were tailored to fit the photosystem I donor and acceptor sides, respectively. Furthermore, we show that by coupling the developed biocathode with a BiVO4/CoP photoanode, a power output of 25 µW/cm2 can be achieved. We further show that our configuration can minimize the damaging effect of H2O2 by two different pathways, oxidation at the photoanode or reduction by the polymethylene blue layer at the biocathode.

Modelling Photosynthesis with ZnII -Protoporphyrin All-DNA G-Quadruplex/Aptamer Scaffolds.

All-DNA scaffolds act as templates for the organization of photosystem I model systems. A series of DNA templates composed of ZnII -protoporphyrin IX (ZnII PPIX)-functionalized G-quadruplex conjugated to the 3'- or 5'-end of the tyrosinamide (TA) aptamer and ZnII PPIX/G-quadruplex linked to the 3'- and 5'-ends of the TA aptamer through a four-thymidine bridge. Effective photoinduced electron transfer (ET) from ZnII PPIX/G-quadruplex to bipyridinium-functionalized tyrosinamide, TA-MV2+ , bound to the TA aptamer units is demonstrated. The effectiveness of the primary ET quenching of ZnII PPIX/G-quadruplex by TA-MV2+ controls the efficiency of the generation of TA-MV+. . The photosystem-controlled formation of TA-MV+. by the different photosystems dictates the secondary activation of the ET cascade corresponding to the ferredoxin-NADP+ reductase (FNR)-catalysed reduction of NADP+ to NADPH by TA-MV+. , and the sequestered alcohol dehydrogenase catalysed reduction of acetophenone to 1-phenylethanol by NADPH.

Photosensitized H2 Evolution and NADPH Formation by Photosensitizer/Carbon Nitride Hybrid Nanoparticles.

The broadband C3N4 semiconductor absorbs in the UV region, λ = 330-380 nm, a feature limiting its application for light-to-energy conversion. The unique surface adsorption properties of C3N4 allow, however, the binding of a photosensitizer, operating in the visible-solar spectrum to the surface of C3N4. Coupling of the energy levels of the photosensitizer with the energy levels of C3N4 allows effective photoinduced electron-transfer quenching and subsequent charge separation in the hybrid structures. Two methods to adsorb a photosensitizer on the C3N4 nanoparticles are described. One is exemplified by the adsorption of Zn(II)-protoporphyrin IX on C3N4 using π-π interactions. The second method utilizes the specific binding interactions of single-stranded nucleic acids on C3N4 and involves the binding of a Ru(II)-tris-bipyridine-modified nucleic acid on the C3N4 nanoparticles. Effective electron-transfer quenching of the photoexcited photosensitizers by C3N4 proceeds in the two hybrid systems. The two hybrid photosystems induce the effective photosensitized reduction of N,N'-dimethyl-4,4'-bipyridinium, MV2+, to MV+•, in the presence of Na2EDTA as a sacrificial electron donor. The generation of MV+• is ca. 5-fold higher as compared to the formation of MV+• in the presence of the photosensitizer alone (in the absence of C3N4). The effective generation of MV+• in the photosystems is attributed to the efficient quenching of the photosensitizers, followed by effective charge separation of the electrons in the conduction band of C3N4 and the holes in the oxidized photosensitizer. The subsequent transfer of the conduction-band electrons to MV2+ and the oxidation of Na2EDTA by the oxidized photosensitizers lead to the effective formation of MV+•. The photogenerated MV+• by the two hybrid photosystems is used to catalyze H2 evolution in the presence of Pt nanoparticle catalysts and to mediate the reduction of NADP+ to NADPH, in the presence of ferredoxin-NADP+ reductase, FNR. The ability to couple the photogenerated NADPH to drive NADP+-dependent biocatalytic transformations is demonstrated.

Artificial Photosynthesis with Electron Acceptor/Photosensitizer-Aptamer Conjugates.

Sequence-specific aptamers act as functional scaffolds for the assembly of photosynthetic model systems. The Ru(II)-tris-bipyridine photosensitizer is conjugated by different binding modes to the antityrosinamide aptamer to yield a set of photosensitizer-aptamer binding scaffolds. The N-methyl-N'-(3-aminopropane)-4,4'-bipyridinium electron acceptor, MV2+, is covalently linked to tyrosinamide, TA, to yield the conjugate TA-MV2+. The tyrosinamide unit in TA-MV2+ acts as a ligand for anchoring TA-MV2+ to the Ru(II)-tris-bipyridine-aptamer scaffold, generating the diversity of photosensitizer-aptamer/electron acceptor supramolecular conjugates. Effective electron transfer quenching in the photosynthetic model systems is demonstrated, and the quenching efficiencies are controlled by the structural features of the conjugates. The redox species generated by the photosensitizer-aptamer/electron acceptor supramolecular systems mediate the ferredoxin-NADP+ reductase, FNR, catalyzed synthesis of NADPH, and the Pt-nanoparticle-catalyzed evolution of hydrogen (H2). The novelty of the study rests on the unprecedented use of aptamer scaffolds as functional units for organizing photosynthetic model systems.