For more than a hundred years, scientists have been struggling with a paradox: the key molecules of life come in two perfectly symmetrical mirror versions, described as "left" or "right," but living organisms only retain one. Amino acids are almost exclusively "left-handed," sugars "right-handed," and so on. This preference, called homochirality, has until now remained without a simple explanation.
Today, an Israeli team proposes a clue from the quantum world: the spin of electrons.
The researchers, led by Yossi Paltiel and Ron Naaman, discovered that when electrons pass through mirror molecules, their spin interacts differently with each form. This difference only appears when the molecules are in motion or involved in reactions. At equilibrium, the two versions remain identical, but in dynamics, their behavior is no longer perfectly mirrored.
The two mirror forms of an amino acid.
Image Wikimedia This subtle bias, though tiny, could accumulate over time. If one version of a molecule interacts slightly more efficiently with its environment thanks to spin influence, it can gain an advantage over the other during chemical reactions or transport processes. Over long periods, this slight imbalance could explain how a single form became dominant in biology.
The study, published in
Science Advances, challenges the idea that mirror molecules have perfectly symmetrical effects.
These results build bridges between physics, chemistry, and biology. They indicate that quantum properties, such as spin, may have influenced molecular evolution from the very beginnings of life.
Electron spin
Spin is a quantum property of electrons, often compared to a rotation around themselves. It can be oriented "up" or "down," and this orientation influences magnetic interactions. In materials, spin plays a key role in phenomena such as magnetism or spintronics. But its involvement in biology is more recent.
The study shows that when electrons pass through an asymmetric (chiral) molecule, their spin preferentially aligns in one direction depending on the molecule's shape, breaking the mirror symmetry. This effect, called "spin-induced chirality," had already been observed in simple systems, but never before in the realm of biology.
This discovery indicates that spin could be a vector of asymmetry at the molecular level.