This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Just like gloves, molecules can have left- and right-handed—or mirror-image—forms. Because the left- and right-handed molecules are still very similar, scientists have had trouble separating them from each other.
But a team of researchers in Japan might have found a solution. In their new study, published in Science Advances, the team successfully separated an equal mixture of mirror-image molecules created by replacing hydrogen with deuterium in one form. Researchers sometimes replace hydrogen in certain molecules with deuterium (H/D substitution).
Deuterium is an isotope of hydrogen that contains both a proton and a neutron in its nucleus. Switching out the hydrogen in some molecules can change the properties slightly, making the altered molecule useful in applications ranging from drug development to material engineering. Yet, these mirror-image molecules, also referred to as enantiomers, are so similar that standard separation methods struggle to separate them.
The authors of the new study write, "Although recent advances in spectroscopic techniques, including anisotropic NMR in chiral oriented media, have enabled the enantiodiscrimination of certain isotopically chiral molecules, these methods are primarily analytical and do not offer a practical solution for the preparative resolution of such compounds." Some attempts at separation were unreliable, and specialized chromatography generally provided weak separation. One attempt was more successful but required 65 passes through a recycling chromatography system. Practical separation of H/D substitution compounds has so far remained elusive.
This has limited researchers to mainly making these molecules through carefully controlled synthesis rather than by separating existing mixtures. The team involved in the new study got the idea to take advantage of a subtle difference in the bond dissociation energy between hydrogen and deuterium with carbon, which they described as "the only energetic handle available for stereochemical discrimination." The method combined a catalyst's preference for molecular orientation with the greater difficulty of breaking carbon–deuterium bonds than carbon–hydrogen bonds. This led them to find a ruthenium-based catalyst that selectively reacted with one mirror-image form faster than the other.
They tested the ruthenium catalyst on equal mixtures of isotope-created mirror images of a compound called 2-ethylnaphthalene, using a reaction that adds a nitrogen-containing group. A test with another molecule separated one form 83 times faster, indicating a large difference in reaction rates between the two forms. The recovered material was estimated to contain more than 99.9% of one mirror-image form.
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