This mirror-image phenomenon -- known as chirality or"handedness" -- has captured the imagination of a UCLAresearch group led by Thomas G. Mason, a professor of chemistry andphysics and a member of the California NanoSystems Institute atUCLA. Mason has been exploring how and why chirality arises, and hisnewest findings on the physical origins of the phenomenon werepublished May 1 in the journal Nature Communications. "Objects like our hands are chiral, while objects like regulartriangles are achiral , meaning they don't have a handedness to them," said Mason,the senior author of the study. |
"Achiral objects can be easilysuperimposed on top of one another." Why many of the important functional molecules in our bodies almostalways occur in just one chiral form when they could potentiallyexist in either is a mystery that has confounded researchers foryears. "Our bodies contain important molecules like proteins thatoverwhelmingly have one type of chirality," Mason said."The other chiral form is essentially not found. I find thatfascinating. We asked, 'Could this biological preference of aparticular chirality possibly have a physical origin?'" In addressing this question, Mason and his team sought to discoverhow chirality occurs in the first place.
Their findings offer newinsights into how the phenomenon can arise spontaneously, even with achiral building-blocks. Mason and his colleagues used a manufacturing technique calledlithography, which is the basis for making computer chips, to makemillions 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 ofletters of the alphabet. Using optical microscopy, the researchers then studied very densesystems of these lithographic triangular particles. To theirsurprise, they discovered that the achiral triangles spontaneouslyarranged themselves to form two-triangle"super-structures," with each super-structure exhibitinga particular chirality. In the image that accompanies this article, the colored outlines inthe field of triangles indicate chiral super-structures havingparticular orientations.
So what is causing this phenomenon to occur? Entropy, says Mason.His group has shown for the first time that chiral structures canoriginate from physical entropic forces acting on uniform achiralparticles. "It's quite bizarre," Mason said. "You're startingwith achiral components -- triangles -- which undergo Brownianmotion and you end up with the spontaneous formation ofsuper-structures that have a handedness or chirality. I would neverhave anticipated that in a million years." Entropy is usually thought of as a disordering force, but thatdoesn't capture its subtler aspects.
In this case, when thetriangular particles are diffusing and interacting at very highdensities on a flat surface, each particle can actually maximizeits "wiggle room" by becoming partially ordered into aliquid 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 -- entropyand particle shape -- are enough to cause chirality to appearspontaneously in dense systems," Mason said. "In my 25years of doing research, I never thought that I would see chiralityoccur in a system of achiral objects driven by entropicforces." As for the future of this research, "We are very interested tosee what happens with other shapes and if we can eventually controlthe chiral formations that we see occurring herespontaneously," he said. "To me, it's intriguing, because I think about the chiralpreference in biology," Mason added. "How did this chiralpreference happen? What are the minimum ingredients for that tooccur? We're learning some new physical rules, but the story inbiology is far from complete.
We have added another chapter to thestory, and I'm amazed by these findings." To learn more, a message board accompanies the publication inNature Communications, an online journal, as a forum forinteractive discussion. This research was funded by the University of California. Kun Zhao,a postdoctoral researcher in Mason's laboratory, made many keycontributions, including fabricating the triangle particles,creating the two-dimensional system of particles, performing theoptical microscopy experiments, carrying out extensiveparticle-tracking analysis and interpreting the results. Along with Mason, co-author Robijn Bruinsma, a UCLA professor oftheoretical physics and a member of the California NanoSystemsInstitute at UCLA, contributed to the understanding of the chiralsymmetry breaking and the liquid crystal phases.
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