02.07.2018 change 02.07.2018

Institute of Physics PAS: A new method of separation of chiral molecules

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A new method of separation of dextrorotatory and levorotary chiral molecules (molecules that are mirror images of each other) has been developed by an international group of scientists, including researchers from the Institute of Physics of the Polish Academy of Sciences. The results of their research have been published in Science.

An international team of scientists, whose member is Prof. Lech Tomasz Baczewski from the Institute of Physics of the Polish Academy of Sciences in Warsaw, has developed a new way of separating enantiomers. This is a mixture of dextrorotatory and levorotary chiral molecules - chemical molecules with identical composition, which are mirror images of each other, like the right and left hand. Separation of enantiomers is a very important issue for chemists, biologists as well as the pharmaceutical industry.

The results of their research have been published in the prestigious journal Science (DOI:10.1126/science.aar4265), and Institute of Physics PAS informed about them in a press release.

The tragic history of headache and nausea drug called Thalidomid from the mid-twentieth century shows the significance of the enantiomer problem for the pharmaceutical industry. Only after a series of adverse side effects of this drug that occurred in pregnant patients and their children born with severe developmental defects, research showed that dextrorotatory chiral molecules are very dangerous to human health - while the same molecules, but levorotatory, are an effective drug. Since that time, huge financial resources have been spent on the development of an effective method of separation of enantiomers, i.e. the separation of dextrorotatory and levorotary molecules.

In the study published in Science, the authors show for the first time that such an effective separation of enantiomers can be achieved with a specially prepared magnetic nanostructure. The observed separation effect does not depend on the type of molecules - the same separation efficiency has been found for amino acids, oligopeptides and DNA structures. This versatility is an extremely important advantage of this method for the pharmaceutical industry, because it allows to replace the currently used, very expensive separation columns, which in addition must be constructed separately for each molecule type.

In their previous work, members of the team described the so-called CISS (chiral-induced spin selectivity) mechanism. It allows to reorient the direction of magnetization in ferromagnetics carried out only by adsorption of chiral molecules - without applying electric current or magnetic field. Thanks to spin-selective electron transfer through a layer of adsorbed molecules to the ferromagnetic below, the ferromagnetic also becomes spin polarized - and this determines the direction of magnetization.

The next stage of research was the discovery of a new, very important phenomenon: the possibility of separating enantiomers through a thin layer of ferromagnetic with magnetization perpendicular to the surface. It turns out that the redistribution of charge in the chiral molecule causes the dependence of the spin orientation on the direction of the molecule`s twist. The effect opposite to that demonstrated in the previous work was used, i.e. specific spin polarization was induced in the ferromagnetic nanostructure with perpendicular magnetic anisotropy and the chiral molecules were adsorbed on it.

Researchers observed that molecules with one twisting direction are adsorbed much faster when the magnetization of the ferromagnetic cobalt layer is directed upwards - while the molecules with reversed twisting tend to adsorb faster when the magnetization is directed downwards. In the presented method, this separation is a result of a magnetic field, but quantum spin-exchange interactions of the chiral molecules with the ferromagnetic nanostructure on which they have been adsorbed.

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