Bacterial extracellular electron transfer components are spin selective.

Metal-reducing bacteria have adapted the ability to respire extracellular solid surfaces instead of soluble oxidants. This process requires an electron transport pathway that spans from the inner membrane, across the periplasm, through the outer membrane, and to an external surface. Multiheme cytochromes are the primary machinery for moving electrons through this pathway. Recent studies show that the chiral-induced spin selectivity (CISS) effect is observable in some of these proteins extracted from the model metal-reducing bacteria, Shewanella oneidensis MR-1. It was hypothesized that the CISS effect facilitates efficient electron transport in these proteins by coupling electron velocity to spin, thus reducing the probability of backscattering. However, these studies focused exclusively on the cell surface electron conduits, and thus, CISS has not been investigated in upstream electron transfer components such as the membrane-associated MtrA, or periplasmic proteins such as small tetraheme cytochrome (STC). By using conductive probe atomic force microscopy measurements of protein monolayers adsorbed onto ferromagnetic substrates, we show that electron transport is spin selective in both MtrA and STC. Moreover, we have determined the spin polarization of MtrA to be ∼77% and STC to be ∼35%. This disparity in spin polarizations could indicate that spin selectivity is length dependent in heme proteins, given that MtrA is approximately two times longer than STC. Most significantly, our study indicates that spin-dependent interactions affect the entire extracellular electron transport pathway.

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.

Correlation between Ferromagnetic Layer Easy Axis and the Tilt Angle of Self Assembled Chiral Molecules.

The spin-spin interactions between chiral molecules and ferromagnetic metals were found to be strongly affected by the chiral induced spin selectivity effect. Previous works unraveled two complementary phenomena: magnetization reorientation of ferromagnetic thin film upon adsorption of chiral molecules and different interaction rate of opposite enantiomers with a magnetic substrate. These phenomena were all observed when the easy axis of the ferromagnet was out of plane. In this work, the effects of the ferromagnetic easy axis direction, on both the chiral molecular monolayer tilt angle and the magnetization reorientation of the magnetic substrate, are studied using magnetic force microscopy. We have also studied the effect of an applied external magnetic field during the adsorption process. Our results show a clear correlation between the ferromagnetic layer easy axis direction and the tilt angle of the bonded molecules. This tilt angle was found to be larger for an in plane easy axis as compared to an out of plane easy axis. Adsorption under external magnetic field shows that magnetization reorientation occurs also after the adsorption event. These findings show that the interaction between chiral molecules and ferromagnetic layers stabilizes the magnetic reorientation, even after the adsorption, and strongly depends on the anisotropy of the magnetic substrate. This unique behavior is important for developing enantiomer separation techniques using magnetic substrates.

AFM-Based Spin-Exchange Microscopy Using Chiral Molecules.

Local magnetic imaging at nanoscale resolution is desirable for basic studies of magnetic materials and for magnetic logic and memories. However, such local imaging is hard to achieve by means of standard magnetic force microscopy. Other techniques require low temperatures, high vacuum, or strict limitations on the sample conditions. A simple and robust method is presented for locally resolved magnetic imaging based on short-range spin-exchange interactions that can be scaled down to atomic resolution. The presented method requires a conventional AFM tip functionalized with a chiral molecule. In proximity to the measured magnetic sample, charge redistribution in the chiral molecule leads to a transient spin state, caused by the chiral-induced spin-selectivity effect, followed by the exchange interaction with the imaged sample. While magnetic force microscopy imaging strongly depends on a large working distance, an accurate image is achieved using the molecular tip in proximity to the sample. The chiral molecules' spin-exchange interaction is found to be 150 meV. Using the tip with the adsorbed chiral molecules, two oppositely magnetized samples are characterized, and a magnetic imaging is performed. This method is simple to perform at room temperature and does not require high-vacuum conditions.

Magnetic-related States and Order Parameter Induced in a Conventional Superconductor by Nonmagnetic Chiral Molecules.

Hybrid ferromagnetic/superconducting systems are well-known for hosting intriguing phenomena such as emergent triplet superconductivity at their interfaces and the appearance of in-gap, spin-polarized Yu-Shiba-Rusinov (YSR) states bound to magnetic impurities on a superconducting surface. In this work we demonstrate that similar phenomena can be induced on a surface of a conventional superconductor by chemisorbing nonmagnetic chiral molecules. Conductance spectra measured on NbSe2 flakes over which chiral α-helix polyalanine molecules were adsorbed exhibit, in some cases, in-gap states nearly symmetrically positioned around zero bias that shift with magnetic field, akin to YSR states, as corroborated by theoretical simulations. Other samples show evidence for a collective phenomenon of hybridized YSR-like states giving rise to unconventional, possibly triplet superconductivity, manifested in the conductance spectra by the appearance of a zero bias conductance that diminishes, but does not split, with magnetic field. The transition between these two scenarios appears to be governed by the density of adsorbed molecules.