Magnets sort mirror-image molecules

Life on Earth is asymmetric, right down to the molecular scale. DNA’s double helix, for example, always twists in the same direction. Mirror symmetry is also absent in many drugs, pesticides, and other substances designed to influence biological function, and a molecule’s two mirror-image forms—or enantiomers, as they’re called—can have drastically different effects. The enantiomer of a beneficial drug can have detrimental side effects. A molecule’s two enantiomers can be separated through their interactions with other asymmetric, or chiral, substances, but the specific recipe and ingredients must be tailored to the shape and properties of the molecule in question.

Now Ron Naaman of the Weizmann Institute of Science in Israel, Yossi Paltiel at the Hebrew University of Jerusalem, and their colleagues have discovered a new way to separate enantiomers based on their differential adsorption on a magnetic surface. The same surface works for many molecules, including oligopeptides, short DNA strands, and single amino acids. And curiously, the surface itself is completely achiral.

The method is based on an effect that Naaman and colleagues first noticed almost two decades ago: When electrons move through chiral molecules, their transport is spin dependent, with spin-up electrons preferentially passing through one enantiomer and spin-down electrons moving more freely through the other. Although the theoretical details are still under investigation, the effect has been attributed to coupling between spin and orbital angular momentum.

When a chiral molecule approaches a surface, a portion of its electron density is inevitably drawn to or repelled by the surface. That electron motion, in turn, creates a temporary spin polarization: an excess of one spin state at each end of the molecule. Because the spins in a magnetic substrate are also polarized, one enantiomer brings the antiparallel spin in contact with the surface (shown by the blue loop in the figure), and the other brings the parallel spin (red loop). Adsorption of the parallel-spin enantiomer is hindered because of the Pauli exclusion principle.

When they tested the method in the lab, Naaman, Paltiel, and colleagues found that the spin-favored enantiomer adsorbed up to eight times more frequently than its mirror image. Because the molecular spin polarization is only temporary, however, the effect is transient: When left in contact with the magnet for more than a few minutes, both enantiomers adsorb in equal amounts. (K. Banerjee-Ghosh et al., Science, in press, doi:10.1126/science.aar4265 .)