Chiral-Induced Spin Selectivity in Capacitively Coupled Molecules

Chiral Induced Spin Selectivity

Since the initial landmark study on the chiral induced spin selectivity (CISS) effect in 1999, considerable experimental and theoretical efforts have been made to understand the physical underpinnings and mechanistic features of this interesting phenomenon. As first formulated, the CISS effect refers to the innate ability of chiral materials to act as spin filters for electron transport; however, more recent experiments demonstrate that displacement currents arising from charge polarization of chiral molecules lead to spin polarization without the need for net charge flow. With its identification of a fundamental connection between chiral symmetry and electron spin in molecules and materials, CISS promises profound and ubiquitous implications for existing technologies and new approaches to answering age old questions, such as the homochiral nature of life. This review begins with a discussion of the different methods for measuring CISS and then provides a comprehensive overview of molecules and materials known to exhibit CISS-based phenomena before proceeding to identify structure-property relations and to delineate the leading theoretical models for the CISS effect. Next, it identifies some implications of CISS in physics, chemistry, and biology. The discussion ends with a critical assessment of the CISS field and some comments on its future outlook.

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.