Achiral triangles form chiral super-structures. Colored patches represent parallelogram outlines around pairs of triangles that have formed chiral super-structures. Parallelograms having different “handedness” and orientations are color-coded and superimposed over each other. Credit: Thomas G. Mason and Kun Zhao |
The
overwhelming majority of proteins and other functional molecules in our
bodies display a striking molecular characteristic: They can exist in
two distinct forms that are mirror images of each other, like your right
hand and left hand. Surprisingly, each of our bodies prefers only one
of these molecular forms.
This
mirror-image phenomenon—known as chirality or “handedness”—has captured
the imagination of a UCLA research group led by Thomas G. Mason, a
professor of chemistry and physics and a member of the California
NanoSystems Institute at UCLA.
Mason
has been exploring how and why chirality arises, and his newest
findings on the physical origins of the phenomenon were published May 1
in the journal Nature Communications.
“Objects like our hands are chiral, while objects like regular triangles are achiral,
meaning they don’t have a handedness to them,” said Mason, the senior
author of the study. “Achiral objects can be easily superimposed on top
of one another.”
Why
many of the important functional molecules in our bodies almost always
occur in just one chiral form when they could potentially exist in
either is a mystery that has confounded researchers for years.
“Our
bodies contain important molecules like proteins that overwhelmingly
have one type of chirality,” Mason said. “The other chiral form is
essentially not found. I find that fascinating. We asked, ‘Could this
biological preference of a particular chirality possibly have a physical
origin?'”
In
addressing this question, Mason and his team sought to discover how
chirality occurs in the first place. Their findings offer new insights
into how the phenomenon can arise spontaneously, even with achiral building-blocks.
Mason
and his colleagues used a manufacturing technique called lithography,
which is the basis for making computer chips, to make millions of
microscale particles in the shape of achiral triangles. In the past,
Mason has used this technique to “print” particles in a wide variety of
shapes, and even in the form of letters of the alphabet.
Using
optical microscopy, the researchers then studied very dense systems of
these lithographic triangular particles. To their surprise, they
discovered that the achiral triangles spontaneously arranged themselves
to form two-triangle “super-structures,” with each super-structure
exhibiting a particular chirality.
In
the image that accompanies this article, the colored outlines in the
field of triangles indicate chiral super-structures having particular
orientations.
So
what is causing this phenomenon to occur? Entropy, says Mason. His
group has shown for the first time that chiral structures can originate
from physical entropic forces acting on uniform achiral particles.
“It’s
quite bizarre,” Mason said. “You’re starting with achiral
components—triangles—which undergo Brownian motion and you end up with
the spontaneous formation of super-structures that have a handedness or
chirality. I would never have anticipated that in a million years.”
Entropy
is usually thought of as a disordering force, but that doesn’t capture
its subtler aspects. In this case, when the triangular particles are
diffusing and interacting at very high densities on a flat surface, each
particle can actually maximize its “wiggle room” by becoming partially
ordered into a liquid crystal (a phase of matter between a liquid and a
solid) made out of chiral super-structures of triangles.
“We
discovered that just two physical ingredients—entropy and particle
shape—are enough to cause chirality to appear spontaneously in dense
systems,” Mason said. “In my 25 years of doing research, I never thought
that I would see chirality occur in a system of achiral objects driven
by entropic forces.”
As
for the future of this research, “We are very interested to see what
happens with other shapes and if we can eventually control the chiral
formations that we see occurring here spontaneously,” he said.
“To
me, it’s intriguing, because I think about the chiral preference in
biology,” Mason added. “How did this chiral preference happen? What are
the minimum ingredients for that to occur? We’re learning some new
physical rules, but the story in biology is far from complete. We have
added another chapter to the story, and I’m amazed by these findings.”
To
learn more, a message board accompanies the publication in Nature
Communications, an online journal, as a forum for interactive
discussion.
This
research was funded by the University of California. Kun Zhao, a
postdoctoral researcher in Mason’s laboratory, made many key
contributions, including fabricating the triangle particles, creating
the two-dimensional system of particles, performing the optical
microscopy experiments, carrying out extensive particle-tracking
analysis and interpreting the results.
Along
with Mason, co-author Robijn Bruinsma, a UCLA professor of theoretical
physics and a member of the California NanoSystems Institute at UCLA,
contributed to the understanding of the chiral symmetry breaking and the
liquid crystal phases.
