that are twisted are ubiquitous in nature, and have important
consequences in biology, chemistry, physics and medicine. Some molecules
have unique and technologically useful optical properties; the
medicinal properties of drugs depend on the direction of the twist; and
within us—think of the double helix—twisted DNA can interact with
twisting is called chirality and researchers at Case Western Reserve
University have found they can use a macroscopic blunt force to impose
and induce a twist in an otherwise non-chiral molecule.
Their new “top-down” approach is described in the Dec. 2 issue of Physical Review Letters.
key is that we used a macroscopic force to create chirality down to the
molecular level,” said Charles Rosenblatt, professor of physics at Case
Western Reserve and the senior author on the paper. Rosenblatt started
the research with no application in mind. He simply wanted to see if it
could be done—essentially scientific acrobatics.
he points out, since antiquity chirality has played a role in health,
energy, technology and more—but until now, chirality always has been a
bottom-up phenomenon. This new top-down approach, if it can be scaled
up, could lead to custom designed chirality—and therefore desired
properties—in all kinds of things.
worked with post-doctoral researcher Rajratan Basu, graduate student
Joel S. Pendery, and professor Rolfe G. Petschek, of the physics
department at Case Western Reserve, and Chemistry Professor Robert P.
Lemieux of Queen’s University, Kingston, Ontario.
isn’t as simple as a twist in a material. More precisely, a chiral
object can’t be superimposed on its mirror image. In a “thought
experiment,” if one’s hand can pass through a mirror (like Alice Through
the Looking Glass), the hand cannot be rotated so that it matches its
mirror image. Therefore one’s hand is chiral.
on the twist, scientists define chiral objects as left-handed and
right-handed. Objects that can superimpose themselves on their mirror
image, such as a wine goblet, are not chiral.
optics, chiral molecules rotate the polarization of light—the direction
depends on whether the molecules are left-handed or right-handed.
Liquid crystal computer and television screen manufacturers take
advantage of this property to enable you to clearly see images from an
the drug industry, chirality is crucial. Two drugs with the identical
chemical formula have different uses. Dextromethorphan, which is
right-handed, is a cough syrup and levomethorphan, which is lefthanded,
is a narcotic painkiller.
reason for the different effects? The drugs interact differently with
biomolecules inside us, depending on the biomolecules’ chirality.
meeting with Lemieux at a conference, the researchers invented a method
to create chirality in a liquid crystal at the molecular level.
treated two glass slides so that cigar-shaped liquid crystal molecules
would align along a particular direction. They then created a thin cell
with the slides, but rotated the two alignment directions by
approximately a 20-degree angle.
20-degree difference caused the molecules’ orientation to undergo a
right-handed helical rotation, like a standard screw, from one side to
the other. This is the imposed chiral twist.
twist, however, is like a tightened spring and costs energy to
maintain. To reduce this cost, some of the naturally left-handed
molecules in the crystal became right-handed. That’s because,
inherently, right-handed molecules give rise to a macroscopic
right-handed twist, Rosenblatt explained. This shift of molecules from
left-handed to right-handed is the induced chirality.
the law of entropy suggests there would be nearly identical numbers of
left-handed and right-handed molecules, in order to keep total energy
cost at a minimum, the right-handed molecules outnumbered the left, he
test for chirality, the researchers applied an electrical field
perpendicular to the molecules. If there were no chirality, there would
be nothing to see. If there were chirality, the helical twist would
rotate in proportion to the amount of right-handed excess.
They observed a modest rotation, which became larger when they increased the twist.
“The effect was occurring everywhere in the cell, but was strongest at the surface,” Rosenblatt said.
have built chirality into optical materials, electrooptic devices, and
more by starting at the molecular level. But the researchers are not
aware of other techniques that use a macroscopic force to bring chiralty
down to molecules.
The researchers are continuing to investigate ways this can be done.