Most biomolecules come in either left- or right-handed forms, known as handedness or chirality. Interestingly, our bodies prefer one chirality over the other. For example, amino acids generally come in left-handed form and sugars in right-handed form.
Thomas G. Mason, professor of chemistry and physics and a member of the California NanoSystems Institute at University of California in Los Angeles (UCLA) and colleagues, including researchers from Baylor College of Medicine, have been studying this mirror-image property to determine the physical origins of this phenomenon. More information is available in the journal Nature Communications (May 1, 2013).
According to Mason, “Objects like our hands are chiral, while objects like regular triangles are achiral, meaning they don’t have a handedness to them. Achiral objects can be easily superimposed on top of one another.” It has been a mystery as to why our bodies prefer one chiral form over another. Mason and colleagues wanted to know if this preference had a physical origin.
The first approach in their research was to determine how chirality occurs to begin with. The researchers have found that this phenomenon can come about spontaneously, even when starting with achiral building-blocks.
The researchers used a technique known as lithography, which is used to make computer chips, and made millions of microscale particles in the shape of triangles. Then, they used optical microscopy to observe the achiral triangles spontaneously arrange themselves to form two-triangle “super-structures.” Each super-structure exhibited a particular handedness. Mason and colleagues concluded that chirality can originate from physical entropic forces acting on uniform achiral particles.
The triangles apparently undergo Brownian motion and spontaneously form these super-structures, which express chirality. Mason notes, “I would never have anticipated that in a million years.”
One generally thinks of entropy as being a disordering force, however, it is much more complicated than that. Mason points out, “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.” The researchers discovered that it takes two physical components, entropy and particle shape. 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.”
Next on the agenda will be to study other shapes and see if they can control chiral formations.
Mason notes, “To me, it’s intriguing, because I think about the chiral preference in biology. 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.”