Chirality-Induced Magnetization and Its Implications for RNA Homochirality

TL;DR

The study investigates how chirality-induced spin selectivity (CISS) can drive RNA homochirality in plausible prebiotic environments by coupling chiral molecules to magnetic surfaces. It combines magneto-optical Kerr effect (MOKE) imaging, Hall measurements, and quantitative NMR to assess how temperature affects chiral magnetization and RAO stability. The main finding is that RAO adsorption increases coercivity on magnetite in a temperature-dependent manner, consistent with vibronic (phonon-assisted) CISS, while RAO yield in prebiotic synthesis also rises with temperature due to greater thermodynamic stability, as shown by NMR. A complementary microscopic model predicts a linear-in-temperature correction to the magnetic anisotropy energy, providing a plausible mechanism for robust spin-controlled processes across environmental temperature ranges relevant to early Earth, thereby strengthening a spin-based route to life's homochirality.

Abstract

The single-handedness of biomolecules is a characteristic feature of life on Earth, yet achieving and maintaining a homochiral prebiotic network is a significant challenge. In our previous studies, we reached homochirality in an RNA precursor, ribo-aminooxazoline (RAO), using magnetized magnetite surfaces as chiral reagents, due to the chiral-induced spin selectivity (CISS) effect. We further demonstrated that RAO can induce net magnetization on previously demagnetized magnetite, indicating a self-reinforcing feedback between chiral molecules and magnetic surfaces. However, these processes depend on spin interactions that generally weaken at higher temperatures, raising concerns about the robustness of our mechanisms in natural settings. Additionally, the temperature dependence of CISS remains poorly understood and highly debated. To address these questions, we investigated the temperature dependence of chirality-induced magnetization. Contrary to classical expectations, we observed a significant increase in the magnetic coercivity with increasing temperatures, suggesting a phonon-assisted process. Our results support a vibronic contribution to CISS and indicate that spin-controlled processes leading to RNA homochirality can occur reliably and effectively over a range of temperatures likely present in prebiotic environments.

Figures (20)

• Figure 1: Chiral molecules can alter the magnetic properties of ferromagnetic surfaces through spin-exchange interactions, arising from transient spin splitting in a chiral potential due to the chiral-induced spin selectivity (CISS) effect. Spin polarization across a chiral molecule due to CISS is higher at higher temperatures, leading to an increase in chirality-induced magnetization and enhanced ferromagnetic order at higher temperatures: $\mu_H > \mu_C$.
• Figure 2: a Enantiopure RAO crystals were drop-cast onto a Si/Ni(50 nm)/Au(5 nm) thin film, forming a sample imaged with a MOKE microscope. In this setup, linearly polarized light is used to image the surface magnetization of the sample. The rotation of the polarization of the incident light corresponds to the surface magnetization. b Magnetic hysteresis loops are obtained by sweeping the magnetic field and averaging the intensity of regions far from (top) and beneath (bottom) homochiral L-RAO crystals. As temperature increases, the loops beneath the crystals widen, indicating an increased coercive field, while the bare surface’s coercive field remains constant. The vertical line at 3.85 mT is chosen to illustrate the difference in magnetization images at different temperatures, as seen in panels e and f. c Coercive fields as a function of temperature for regions beneath (red) and far from (gray) the L-RAO crystals. The coercive field beneath the crystals is 2 mT higher at room temperature and increases linearly with rising temperature. d Optical image of the sample. The area covered by L-RAO crystals is outlined by a dashed white line, and regions beneath the crystals and far from the crystals, where magnetization loops were taken, are marked in red/gray, respectively. e Magnetic image at 20°C and a 3.85 mT magnetic field, showing all domains flipped. f Magnetic image at 80°C and a 3.85 mT magnetic field, where some domains beneath the crystals remain unflipped.
• Figure 3: a The prebiotic synthesis of pentose-aminooxazolines produces four pairs of stereoisomers: ribose-, arabinose-, xylose-, and lyxose-aminooxazolines. While the natural ribose form is the major product, it is produced within a mixture. The arabinose form is nearly as abundant as RAO, with a ratio of RAO to AAO around 1.2 at 20°C after a day of incubation. b, c Concentrations of RAO (blue dots), AAO (red diamonds), and XAO (green squares) after 2.5 hours b and 24 hours c of incubation of glyceraldehyde and 2-aminooxazole as a function of temperature. The RAO to AAO ratio (second y axis, dashed line) increases significantly as both aminooxazolines begin degrading via hydrolysis. At 65°C, after a day of incubation, the ratio increases to 1.5, marking a 23% increase compared to 20°C. Therefore, at higher temperatures, the natural isomer RAO could be enriched due to the higher thermodynamic stability of RAO.
• Figure 4: a The temperature dependence of CISS can be interpreted with a thermodynamic picture. In this model, the work done by chiral molecules—reducing entropy by filtering accessible spin states—is compensated by heat supplied to the molecules in the form of phonons, thereby enhancing the efficiency of this “chiral engine.” b Magnon energy dispersion relation as a function of (left) coupling strength between spin and vibrational modes, q, for five different temperatures between 1 K and 1,500 K and (b) temperature for two different values of q. Here, the spin-lattice coupling $A_z=1/4$, axial anisotropy $I_0=-1/8$ in units of $J=80$ meV.
• Figure S1: a Magneto-optical Kerr effect microscope (MOKE) setup is displayed. The magnetic surface is imaged by a Zeiss objective, and an in-plane magnetic field was generated by an electromagnet placed around the imaging plane of the microscope. The yellow arrows represent the optical path. b A printed circuit board (PCB) with a Peltier element for heating connected to c a PCB sample holder. The sample is glued to a heat-conducting surface using carbon tape with a Pt thermistor to monitor temperature.