Researchers from the Hebrew University of Jerusalem, University of Southern California, RPTU Kaiserslautern-Landau, Johannes Gutenberg-Universitat Mainz, Ariel University, California Institute of Technology, Uppsala University and the Weizmann Institute have reported a spin-dependent mechanism that may resolve one of the longest-standing questions in science: not only how homochirality emerged, but why a specific handedness was selected.
For more than 150 years, scientists have sought to understand why biological systems exclusively use one enantiomeric form - D-type for RNA and specific handedness for amino acids - despite the near-identical chemical properties of mirror-image molecules. Previous work established that homochirality could arise via enantioselective interactions with magnetic substrates, such as magnetite, through the chirality-induced spin selectivity (CISS) effect. However, this framework did not explain why one enantiomer is ultimately favored over the other.
The recent study introduces a key physical asymmetry rooted in spin dynamics. When electrons traverse chiral molecules, or when such molecules host unpaired electrons, the total angular momentum vector J aligns along an “easy axis” determined by spin–orbit coupling and the asymmetric molecular potential. While the magnitude of J remains identical for both enantiomers, its orientation relative to the molecular frame differs.
This difference can be quantified by the angle between J and the molecular electric dipole moment μ. Crucially, this geometric mismatch leads to enantiomer-specific spin behavior during dynamic processes. As a result, the two mirror-image molecules no longer behave as exact opposites in magnitude - contrary to a long-standing assumption - but instead exhibit measurable asymmetries in spin-related interactions.
Experimental measurements, supported by theoretical modeling and ab initio calculations, confirm that these differences manifest in several key processes:
- Spin polarization generated during electron transport differs between enantiomers.
- Interaction efficiencies with magnetic substrates are not identical.
- Dynamic processes, including charge transfer and chemical reactivity, show enantiospecific variation.
Importantly, these effects do not appear in static properties such as energy, where the enantiomers remain equivalent. The asymmetry emerges only under dynamic, spin-involving conditions.
This distinction provides a plausible pathway for symmetry breaking in prebiotic environments. If one enantiomer consistently exhibits slightly higher efficiency in spin-dependent interactions - such as electron transfer or adsorption on magnetic surfaces - these small differences could accumulate over time, ultimately leading to the global dominance of a single handedness.
The results extend the CISS-based model of homochirality by introducing a mechanism that selects not just for chirality, but for a specific enantiomer. In doing so, they suggest that quantum mechanical spin effects, rather than purely chemical considerations, played a decisive role in shaping the molecular architecture of life.
Beyond origins-of-life research, the findings point toward broader implications for spin-dependent enantiospecific processes, with potential relevance to molecular electronics, catalysis, and chiral device design.