Probing the design principles of photosynthetic systems through fluorescence noise measurement.

Elucidating the energetic processes which govern photosynthesis, the engine of life on earth, are an essential goal both for fundamental research and for cutting-edge biotechnological applications. Fluorescent signal of photosynthetic markers has long been utilised in this endeavour. In this research we demonstrate the use of fluorescent noise analysis to reveal further layers of intricacy in photosynthetic energy transfer. While noise is a common tool analysing dynamics in physics and engineering, its application in biology has thus far been limited. Here, a distinct behaviour in photosynthetic pigments across various chemical and biological environments is measured. These changes seem to elucidate quantum effects governing the generation of oxidative radicals. Although our method offers insights, it is important to note that the interpretation should be further validated expertly to support as conclusive theory. This innovative method is simple, non-invasive, and immediate, making it a promising tool to uncover further, more complex energetic events in photosynthesis, with potential uses in environmental monitoring, agriculture, and food-tech.

Chiral Molecular Coating of a LiNiCoMnO2 Cathode for High-Rate Capability Lithium-Ion Batteries.

The growing demand for energy has increased the need for battery storage, with lithium-ion batteries being widely used. Among those, nickel-rich layered lithium transition metal oxides [LiNi1-x-yCoxMnyO2 NCM (1 - x - y > 0.5)] are some of the promising cathode materials due to their high specific capacities and working voltages. In this study, we demonstrate that a thin, simple coating of polyalanine chiral molecules improves the performance of Ni-rich cathodes. The chiral organic coating of the active material enhances the discharge capacity and rate capability. Specifically, NCM811 and NCM622 electrodes coated with chiral molecules exhibit lower voltage hysteresis and better rate performance, with a capacity improvement of >10% at a 4 C discharge rate and an average improvement of 6%. We relate these results to the chirally induced spin selectivity effect that enables us to reduce the resistance of the electrode interface and to reduce dramatically the overpotential needed for the chemical process by aligning the electron spins.

Probing chiral discrimination in biological systems using atomic force microscopy: The role of van der Waals and exchange interactions.

We analyze from a theoretical perspective recent experiments where chiral discrimination in biological systems was established using Atomic Force Microscopy (AFM). Even though intermolecular forces involved in AFM measurements have different origins, i.e., electrostatic, bonding, exchange, and multipole interactions, the key molecular forces involved in enantiospecific biorecognition are electronic spin exchange and van der Waals (vdW) dispersion forces, which are sensitive to spin-orbit interaction (SOI) and space-inversion symmetry breaking in chiral molecules. The vdW contribution to chiral discrimination emerges from the inclusion of SOI and spin fluctuations due to the chiral-induced selectivity effect, a result we have recently demonstrated theoretically. Considering these two enantiospecific contributions, we show that the AFM results regarding chiral recognition can be understood in terms of a simple physical model that describes the different adhesion forces associated with different electron spin polarization generated in the (DD), (LL), and (DL) enantiomeric pairs, as arising from the spin part of the exchange and vdW contributions. The model can successfully produce physically reasonable parameters accounting for the vdW and exchange interaction strength, accounting for the chiral discrimination effect. This fact has profound implications in biorecognition where the relevant intermolecular interactions in the intermediate-distance regime are clearly connected to vdW forces.

Magneto-optical imaging of magnetic-domain pinning induced by chiral molecules.

Chiral molecules have the potential for creating new magnetic devices by locally manipulating the magnetic properties of metallic surfaces. When chiral polypeptides chemisorb onto ferromagnets, they can induce magnetization locally by spin exchange interactions. However, direct imaging of surface magnetization changes induced by chiral molecules was not previously realized. Here, we use magneto-optical Kerr microscopy to image domains in thin films and show that chiral polypeptides strongly pin domains, increasing the coercive field locally. In our study, we also observe a rotation of the easy magnetic axis toward the out-of-plane, depending on the sample's domain size and the adsorption area. These findings show the potential of chiral molecules to control and manipulate magnetization and open new avenues for future research on the relationship between chirality and magnetization.

Nuclear spin effects in biological processes.

Traditionally, nuclear spin is not considered to affect biological processes. Recently, this has changed as isotopic fractionation that deviates from classical mass dependence was reported both in vitro and in vivo. In these cases, the isotopic effect correlates with the nuclear magnetic spin. Here, we show nuclear spin effects using stable oxygen isotopes (16O, 17O, and 18O) in two separate setups: an artificial dioxygen production system and biological aquaporin channels in cells. We observe that oxygen dynamics in chiral environments (in particular its transport) depend on nuclear spin, suggesting future applications for controlled isotope separation to be used, for instance, in NMR. To demonstrate the mechanism behind our findings, we formulate theoretical models based on a nuclear-spin-enhanced switch between electronic spin states. Accounting for the role of nuclear spin in biology can provide insights into the role of quantum effects in living systems and help inspire the development of future biotechnology solutions.

Modification of Weak Localization in Metallic Thin Films Due to the Adsorption of Chiral Molecules.

We perform low-temperature magneto-conductance measurements on Cu and Au thin films with adsorbed chiral molecules and investigate their phase-coherent transport properties. Upon adsorption of chiral molecules, the spin-orbit coupling strength in Cu decreases and the Au films become ferromagnetic as evident from weak localization and antilocalization data. A theoretical model indicates that anisotropy in the molecular tilt angles, provided that the chiral molecules act as magnetic moments, induces a nonvanishing magnetic exchange interaction, causing changes in the spin-orbit coupling strength in Cu and Au. Our work adds a new viewpoint to the plethora of unique phenomena emerging from chiral molecule adsorption on materials.

Optical Activity and Spin Polarization: The Surface Effect.

Chirality ('handedness') is a property that underlies a broad variety of phenomena in nature. Chiral molecules appear in two forms, and each is a mirror image of the other, the two enantiomers. The chirality of molecules is associated with their optical activity, and circular dichroism is commonly applied to identify the handedness of chiral molecules. Recently, the chiral induced spin selectivity (CISS) effect was established, according to which transfer of electrons within chiral molecules depends on the electron's spin. Which spin is preferred depends on the handedness of the chiral molecule and the direction of motion of the electron. Several experiments in the past indicated that there may be a relation between the optical activity of the molecules and their spin selectivity. Here, we show that for a molecule containing several stereogenic axes, when adsorbed on a metal substrate, the peaks in the CD spectra have the same signs for the two enantiomers. This is not the case when the molecules are adsorbed on a nonmetallic substrate or dissolved in solution. Quantum chemical simulations are able to explain the change in the CD spectra upon adsorption of the molecules on conductive and nonconductive surfaces. Surprisingly, the CISS properties are similar for the two enantiomers when adsorbed on the metal substrate, while when the molecules are adsorbed on nonmetallic surface, the preferred spin depends on the molecule handedness. This correlation between the optical activity and the CISS effect indicates that the CISS effect relates to the global polarizability of the molecule.

Ultrafast Coherent Delocalization Revealed in Multilayer QDs under a Chiral Potential.

In recent years, it was found that current passing through chiral molecules exhibits spin preference, an effect known as Chiral Induced Spin Selectivity (CISS). The effect also enables the reduction of scattering and therefore enhances delocalization. As a result, the delocalization of an exciton generated in the dots is not symmetric and relates to the electronic and hole excited spins. In this work utilizing fast spectroscopy on hybrid multilayered QDs with a chiral polypeptide linker system, we probed the interdot chiral coupling on a short time scale. Surprisingly, we found strong coherent coupling and delocalization despite having long 4-nm chiral linkers. We ascribe the results to asymmetric delocalization that is controlled by the electron spin. The effect is not measured when using shorter nonchiral linkers. As the system mimics light-harvesting antennas, the results may shed light on a mechanism of fast and efficient energy transfer in these systems.

The effect of spin exchange interaction on protein structural stability.

Partially charged chiral molecules act as spin filters, with preference for electron transport toward one type of spin ("up" or "down"), depending on their handedness. This effect is named the chiral induced spin selectivity (CISS) effect. A consequence of this phenomenon is spin polarization concomitant with electric polarization in chiral molecules. These findings were shown by adsorbing chiral molecules on magnetic surfaces and investigating the spin-exchange interaction between the surface and the chiral molecule. This field of study was developed using artificial chiral molecules. Here we used such magnetic surfaces to explore the importance of the intrinsic chiral properties of proteins in determining their stability. First, proteins were adsorbed on paramagnetic and ferromagnetic nanoparticles in a solution, and subsequently urea was gradually added to induce unfolding. The structural stability of proteins was assessed using two methods: bioluminescence measurements used to monitor the activity of the Luciferase enzyme, and fast spectroscopy detecting the distance between two chromophores implanted at the termini of a Barnase core. We found that interactions with magnetic materials altered the structural and functional resilience of the natively folded proteins, affecting their behavior under varying mild denaturing conditions. Minor structural disturbances at low urea concentrations were impeded in association with paramagnetic nanoparticles, whereas at higher urea concentrations, major structural deformation was hindered in association with ferromagnetic nanoparticles. These effects were attributed to spin exchange interactions due to differences in the magnetic imprinting properties of each type of nanoparticle. Additional measurements of proteins on macroscopic magnetic surfaces support this conclusion. The results imply a link between internal spin exchange interactions in a folded protein and its structural and functional integrity on magnetic surfaces. Together with the accumulating knowledge on CISS, our findings suggest that chirality and spin exchange interactions should be considered as additional factors governing protein structures.

Controlling photosynthetic energy conversion by small conformational changes.

Control phenomena in biology usually refer to changes in gene expression and protein translation and modification. In this paper, another mode of regulation is highlighted; we propose that photosynthetic organisms can harness the interplay between localization and delocalization of energy transfer by utilizing small conformational changes in the structure of light-harvesting complexes. We examine the mechanism of energy transfer in photosynthetic pigment-protein complexes, first through the scope of theoretical work and then by in vitro studies of these complexes. Next, the biological relevance to evolutionary fitness of this localization-delocalization switch is explored by in vivo experiments on desert crust and marine cyanobacteria, which are both exposed to rapidly changing environmental conditions. These examples demonstrate the flexibility and low energy cost of this mechanism, making it a competitive survival strategy.

Cooperative Effect of Electron Spin Polarization in Chiral Molecules Studied with Non-Spin-Polarized Scanning Tunneling Microscopy.

Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al2O3/Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered self-assembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

Spin-Induced Organization of Cellulose Nanocrystals.

Cellulose nanocrystals (CNCs) are composed of chiral cellulose units, which form chiral nematic liquid crystals in water that, upon drying, self-assemble to more complex spiral chiral sheets. This secondary structure arrangement is found to change with an external magnetic or electric field. Here, we show that one of the basic organization driving forces is electron spin, which is produced as the charge redistributes in the organization process of the chiral building blocks. It is important to stress that the electron spin-exchange interactions supply the original driving force and not the magnetic field per se. The results present the first utilization of the chiral-induced spin selectivity (CISS) effect in sugars, enabling one to regulate the CNC bottom-up fabrication process. Control is demonstrated on the organization order of the CNC by utilizing different magnetization directions of the ferromagnetic surface. The produced spin is probed using a simple Hall device. The measured Hall resistance shows that the CNC sheets' arrangement is affected during the first four hours as long as the CNC is in its wet phase. On introducing the 1,2,3,4-butanetetracarboxylic acid cross-linker into the CNC sheet, the packing density of the CNC helical structure is enhanced, presenting an increase in the Hall resistance and the chiral state.

Metal Organic Spin Transistor.

Organic molecules and specifically bio-organic systems are attractive for applications due to their low cost, variability, environmental friendliness, and facile manufacturing in a bottom-up fashion. However, due to their relatively low conductivity, their actual application is very limited. Chiral metallo-bio-organic crystals, on the other hand, have improved conduction and in addition interesting magnetic properties. We developed a spin transistor using these crystals and based on the chiral-induced spin selectivity effect. This device features a memristor type behavior, which depend on trapping both charges and spins. The spin properties are monitored by Hall signal and by an external magnetic field. The spin transistor exhibits nonlinear drain-source currents, with multilevel controlled states generated by the magnetization of the source. Varying the source magnetization enables a six-level readout for the two-terminal device. The simplicity of the device paves the way for its technological application in organic electronics and bioelectronics.

Chirality Nanosensor with Direct Electric Readout by Coupling of Nanofloret Localized Plasmons with Electronic Transport.

The detection of enantiopurity for small sample quantities is crucial, particularly in the pharmaceutical industry; however, existing methodologies rely on specific chiral recognition elements, or complex optical systems, limiting its utility. A nanoscale chirality sensor, for continuously monitoring molecular chirality using an electric circuit readout, is presented. This device design represents an alternative real-time scalable approach for chiral recognition of small quantity samples (less than 103 adsorbed molecules). The active device component relies on a gold nanofloret hybrid structure, i.e., a high aspect ratio semiconductor-metal hybrid nanosystem in which a SiGe nanowire tip is selectively decorated with a gold metallic cap. The tip mechanically touches a counter electrode to generate a nanojunction, and upon exposure to molecules, a metal-molecule-metal junction is formed. Adsorption of chiral molecules at the gold tip induces chirality in the localized plasmonic resonance at the electrode-tip junction and manifests in an enantiospecific current response.

Dynamic Spin-Controlled Enantioselective Catalytic Chiral Reactions.

Enantioselective catalytic chiral reactions are important to all aspects of life sciences. Here we present the first utilization of the chiral induced spin selectivity (CISS) effect to form, enantioselectively, sp3 chiral centers in catalytic reactions, starting from achiral reagents. The enantiomeric symmetry is broken by affecting spin-controlled different reaction dynamics toward each of the enantiomers, using magnetic substrates. Two catalytic reactions are used for this purpose: a sulfide to sulfoxide oxidation and a Diels-Alder cycloaddition reaction, both catalyzed by hematite (Fe2O3). The proof of concept was evaluated by circular dichroism measurements and by chiral high-performance liquid chromatography techniques. These results provide direct evidence that the directionality of the electron spin can break enantiomeric symmetry, enabling asymmetric catalysis without using chiral reagents, solvents, or catalysts.

Marine cyanobacteria tune energy transfer efficiency in their light-harvesting antennae by modifying pigment coupling.

Photosynthetic light harvesting is the first step in harnessing sunlight toward biological productivity. To operate efficiently under a broad and dynamic range of environmental conditions, organisms must tune the harvesting process according to the available irradiance. The marine cyanobacteria Synechococcus WH8102 species is well-adapted to vertical mixing of the water column. By studying its responses to different light regimes, we identify a new photo-acclimation strategy. Under low light, the phycobilisome (PBS) is bigger, with extended rods, increasing the absorption cross-section. In contrast to what was reported in vascular plants and predicted by Forster resonance energy transfer (FRET) calculations, these longer rods transfer energy faster than in the phycobilisomes of cells acclimated to a higher light intensity. Comparison of cultures grown under different blue light intensities, using fluorescence lifetime and emission spectra dependence on temperature at the range of 4-200 K in vivo, indicates that the improved transfer arises from enhanced energetic coupling between the antenna rods' pigments. We suggest two physical models according to which the enhanced coupling strength results either from additional coupled pathways formed by rearranging rod packing or from the coupling becoming non-classical. In both cases, the energy transfer would be more efficient than standard one-dimensional FRET process. These findings suggest that coupling control can be a major factor in photosynthetic antenna acclimation to different light conditions.

Role of Exchange Interactions in the Magnetic Response and Intermolecular Recognition of Chiral Molecules.

The physical origin of the so-called chirality-induced spin selectivity (CISS) effect has puzzled experimental and theoretical researchers over the past few years. Early experiments were interpreted in terms of unconventional spin-orbit interactions mediated by the helical geometry. However, more recent experimental studies have clearly revealed that electronic exchange interactions also play a key role in the magnetic response of chiral molecules in singlet states. In this investigation, we use spin-polarized closed-shell density functional theory calculations to address the influence of exchange contributions to the interaction between helical molecules as well as of helical molecules with magnetized substrates. We show that exchange effects result in differences in the interaction properties with magnetized surfaces, shedding light into the possible origin of two recent important experimental results: enantiomer separation and magnetic exchange force microscopy with AFM tips functionalized with helical peptides.

Magnetic oriented microparticles preparation.

Generally speaking, reaction platforms involving ferromagnetic surfaces, with a specific magnetic direction, are limited to the two dimensional regime, due to the nature of the magnetic phenomena. Here we show a method for preparing partially coated ferromagnetic microparticles with a distinct magnetic pole. This simple preparation method was presented previously [ 1 ] to demonstrate an application for enantiomeric separation. In this method article we show;•A simple method to a-symmetrically manipulate particle surfaces.•A generic way to synchronize a bare pole of ferromagnetic microparticles.•A simple and generic enantiomer purification technique.

Asymmetric reactions induced by electron spin polarization.

Essential aspects of the chiral induced spin selectivity (CISS) effect and their implications for spin-controlled chemistry and asymmetric electrochemical reactions are described. The generation of oxygen through electrolysis is discussed as an example in which chirality-based spin-filtering and spin selection rules can be used to improve the reaction's efficiency and selectivity. Next the discussion shifts to illustrate how the spin selectivity of chiral molecules (CISS properties) allows one to use the electron spin as a chiral bias for inducing asymmetric reactions and promoting enantiospecific processes. Two enantioselective electrochemical reactions that have used polarized electron spins as a chiral reagent are described; enantioselective electroreduction to resolve an enantiomer from a racemic mixture and an oxidative electropolymerization to generate a chiral polymer from achiral monomers. A complementary approach that has used spin-polarized, but otherwise achiral, molecular films to enantiospecifically associate with one enantiomer from a racemic mixture is also discussed. Each of these reaction types use magnetized films to generate the spin polarized electrons and the enantiospecificity can be selected by choice of the magnetization direction, North pole versus South pole. Possible paths for future research in this area and its compatibility with existing methods based on chiral electrodes are discussed.

Effect of Chiral Molecules on the Electron's Spin Wavefunction at Interfaces.

Kelvin-probe measurements on ferromagnetic thin film electrodes coated with self-assembled monolayers of chiral molecules reveal that the electron penetration from the metal electrode into the chiral molecules depends on the ferromagnet's magnetization direction and the molecules' chirality. Electrostatic potential differences as large as 100 mV are observed. These changes arise from the applied oscillating electric field, which drives spin-dependent charge penetration from the ferromagnetic substrate to the chiral molecules. The enantiospecificity of the response is studied as a function of the magnetization strength, the magnetization direction, and the handedness and length of the chiral molecules. These new phenomena are rationalized in terms of the chiral-induced spin selectivity (CISS) effect, in which one spin orientation of electrons from the ferromagnet penetrates more easily into a chiral molecule than does the other orientation. The large potential changes (>kT at room temperature) manifested here imply that this phenomenon is important for spin transport in chiral spintronic devices and for magneto-electrochemistry of chiral molecules.