Drugs pesticides and fertilizers are
crucial for our survival. However they may contain impurities with harmful side
effects for people, animals and our environment. One of chemical
manufacturing’s biggest challenges is to produce materials that can specifically
perform a pre-designed function and to eliminate contamination of undesired
substances. The CISS effect serves this purpose applying a simple inexpensive
and generic method. But first, what is the CISS effect? Electron transfer is a
process by which energy and information are transferred from one place to
another in living organisms and electronic devices. The chiral-induced
spin selectivity, or CISS, effect has two main actors – chiral molecules and
electron spin. Electrons have two important properties: they carry negative
charge and they have spin. The spin is the angular momentum of the electron
like a spinning top that can rotate clockwise or counter-clockwise referred
to as “spin up” and “spin down”. It behaves like a small magnet. Quantum mechanics
shows that two electrons can occupy the same volume if they have opposite spins.
When two electrons have the same spin they repel each other. How do the chirality
of molecules and spin relate? Chiral molecules have two different
symmetries similar to right or left handedness, which are called enantiomers.
when electrons are moving through chiral systems they behave as if there is a
magnetic field acting on them. This magnetic field interacts with the
magnetic dipole of the electron, namely with the spin of the electron.
Therefore, when an electron is transferred through chiral molecules, one
spin is preferred over the other. Which spin is preferred depends on the
handedness of the molecules – right or left hand-like symmetry and on the
direction of motion. So chiral molecules are like spin filters. This is the CISS
effect. The CISS effect helps to eliminate contamination of undesired substances,
but how? Let’s assume that we start with a neutral molecule which doesn’t have
net charge. When molecules interact with other molecules or a surface, the charge
in the molecule is polarized so that it has a positively charged pole and a
negatively charged pole. This is done by moving electrons from one side of the
molecule to the other. Because of the CISS effect, if the molecule is chiral the
electron motion is accompanied by spin polarization. The spin associated with
each charge depends on the handedness of the molecule. Now let’s discuss the
interaction of chiral molecules with a magnetic surface. In magnetic surfaces,
all the spins are aligned in the same direction. As the molecule approaches the surface it becomes electrically polarized and depending on its
handedness, the spin of the pole pointing towards the surface can be the same or
opposite to the spin of the electrons in the surface. If the spins are opposite
they can occupy the same volume and therefore the molecule and the surface
attract each other. If they are the same, they would repel each other. Hence, if a
solution contains both enantiomers of a given chiral molecule it is possible to
separate the enantiomers by magnetic surface that attracts one and repels the
other. Since life is based on molecules with only one enantiomer,
namely chiral molecules that have specific handedness, it is important to
manufacture drugs of the same handedness. Typically, in a chemical reaction, both
enantiomers are equally produced in a mixture. The undesired enantiomers may
cause side effects disastrous to living beings. The CISS effect helps the chemical and pharmaceutical industry achieve an
inexpensive and generic method for enantiomer separation, enabling a future
of safe drugs pesticides and fertilizers with no negative side effects to people,
animals and our environment. If you’re interested in the CISS effect or the
enantiomer separation technology, contact the inventors Ron Naaman and
Yossi Paltiel.