Elusive electronic effect could explain the origins of chirality for all life on Earth

Magnetic surfaces can sway spin selectivity, resulting in different reaction rates for enantiomers.¹ The discovery of this interaction between mirror molecules and magnetic fields could explain the origins of homochirality on Earth and early life, in particular in prebiotic peptides and RNA.

Most biomolecules exhibit a single handedness – normally natural amino acids have L symmetry, whereas sugars are predominantly D enantiomers. The origins of this phenomenon have puzzled researchers for decades, until an electronic effect emerged as a plausible explanation. This effect, known as chirality-induced spin selectivity (CISS), demonstrates the selection of a specific spin state in electrons travelling through chiral and magnetic materials.

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In this case, combining magnetite, a naturally-occurring magnetic mineral, and ribose aminooxazoline, with a well-known prebiotic precursor of RNA, resulted in surprisingly different CISS interactions for the two enantiomers. The magnetic measurements in mirror molecules differ ‘by a factor of three’, according to this study, which affected spin selectivity and reactivity. This study ‘supports that, if and once homochirality was selected for a pivotal RNA precursor, it could then propagate … to nucleotides, RNA and, potentially, peptides’, explains Claudia Bonfio, who studies the origins of life at the University of Cambridge, UK, and wasn’t involved in this work.

In this work, the rate of reaction for the RNA precursor aminooxazoline differed depending on the enantiomer, explains lead author Ron Naaman from the Weizmann Institute in Israel. Although previous studies suggested opposite outcomes if mirror molecules were exposed to north and south oriented magnets, ‘it was assumed [reaction] rates in both cases were identical’, explains Naaman. Now that’s no longer the case. The only requirement to create an enantiomeric excess ‘is to have a magnetic surface’, he adds.

It was previously presumed that mirror molecules displayed symmetric spin selectivity – presenting with the same strength, but each enantiomer’s spin is in the exact opposite direction. However, in this case the CISS effect seems asymmetric, creating a ‘different magnitude for spin polarisation for different enantiomers’, says John Hudson at Imperial College London, UK, who wasn’t involved in the study. Until now, it was assumed that ‘the degree of spin selectivity would be identical for opposite chiral enantiomers’, he says.

We see a different degree of spin polarisation [between] enantiomers in many molecules, says Naaman. This is established by ‘measuring the magnetoresistance of the molecules’, he adds. Perhaps the most important is the interaction between magnetite and ribose aminooxazoline, where the magnetic measurements in mirror molecules differ ‘by a factor of three’.

This discovery ‘challenges a fundamental assumption in the field’, explains Hudson. While others had previously demonstrated the effects of magnets on the mirror images of the RNA precursor, this paper ‘provides a possible answer to [how] a specific handedness is picked’, adds Naaman. Then, chirality could potentially propagate to RNA and peptides. Other studies by Sasselov, Ozturk and other experts in prebiotic chemistry have already demonstrated that right-handed RNA results in left-handed amino acids, he explains.² However, the emergence of handedness in other biomolecules, including ‘lipids, sugars and other chiral metabolites’, remains a mystery, adds Bonfio.

Maybe more importantly, researchers reported the asymmetry is intrinsically tied to CISS itself: enantiomers exhibit different degrees of spin selectivity, not only opposite effects. The measurements of magnetism supports the differences between enantiomers and ‘were observed across CISS [experiments] in the past two decades’, says Hudson. Computational calculations support this discovery, and ‘the implications of asymmetries in spin selectivity … in the context of the homochirality of biological systems’. Besides being an interesting explanation for enantiomeric excess in early life, it could also become a new tool for chemists to create chiral molecules and materials.