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Self-Propelling Particles May Hold Clue to Life

Ramin Golestanian, a researcher at the Max Planck Institute for Dynamics and Self-Organization, occupies himself with the big questions: How is the thing we call life possible? In particular, he wonders, how can complex subcellular structures so critical for life as we know it form from a soup of enzymes?

“This is basically the Lego-like ingredients [of life],” he says, referring to the fundamental nature of these structures.

As these particles travel, chemical gradients propel certain particles to aggregate together. Over time, these different species of particles can form a singular conglomeration. Image credit: Jaime Agudo-Canalejo and Ramin Golestanian.

He and his University of Oxford postdoctoral scholar, Jaime Agudo-Canalejo, attempt to at least partially answer this question in a recent Physical Review paper. Their study extends the theoretical framework that had been developed to study the movement of so-called Janus particles and extends it to catalytically active particles and enzymes more generally.

“[Janus particles are] primarily a way of synthesizing a colloidal system that can integrate two different tendencies in one particle,” Golestanian explains. In other words, Janus particles—like the Roman god for which they are named—exhibit two-faced behavior, reacting differently when approached from various angles. More concretely, a spherical Janus particle has distinct physical and chemical properties on each hemisphere. In the case of a catalytically active particle, one end might facilitate chemical reactions in the surrounding environment while the other is inert.

Particles in this experiment act similarly to an Alka-Seltzer in water. The tablet dissolves, creating a force pushing the tablet down to the bottom of the glass. (via Giphy)

This lopsided behavior can create an asymmetric force that propels the particle forward, just as the chemical reaction between an Alka-Seltzer tablet and water can shoot a tiny rocket high into the air. Because of their asymmetric interactions, these particles are also highly sensitive to a gradient, an indication of how abruptly the chemical composition of their surroundings changes. In fact, Oxford graduate student Tunrayo Adeleke-Larodo recently worked with Golestanian and Agudo-Canalejo to show that certain enzymes can align themselves using this trick—a tantalizing hint that gradient sensitivity could have something to do with enzyme’s ability to self-assemble.

But where it gets really interesting is when researchers introduce two or more various types of catalytically active particles, especially ones that share reactants or products. This produces a complex system of give-and-take where each particle interacts with the chemical gradients left by another and changes the local environment in its own turn.

As two or more particles influence each other via chemical gradients, they can exhibit a sort of pseudo-interaction that causes them to move around. What’s particularly fascinating about this, says Golestanian, is the fact that these interactions are often non-reciprocal—Particle A’s influence on Particle B may not be the same as Particle B’s influence on Particle A. Golestanian compares this complex behavior to human relationships, where mutual attraction is never a guarantee. This can lead to some very interesting behavior patterns where different types of particles chase each other around in a fluid or even join together to create a single large cluster that pushes itself along like a little motorboat.

 This simulation shows how over time a large number of two different catalytically active particle species can aggregate thanks to gradient sensitivity. Image credit: Jaime Agudo-Canalejo and Ramin Golestanian.

If this feels wrong somehow, it could be because this behavior seems to act in direct contradiction to Newton’s Third Law, which states that for any action there is an equal and opposite reaction. Because of this, the force that two particles exert on each other is almost always equal—the Sun’s gravitational influence on the Earth is precisely equivalent to the Earth’s pull on the Sun, keeping the relative motion of each body stable. From our everyday experience, it just doesn’t seem right that one particle can chase another around indefinitely.

newton satisfying GIF
This gif depicts what Newton’s Third Law looks like with Newton’s Craddle. The red light demonstrates how the force moves in the system. (via Giphy)

However, there is no real contradiction with Newtonian physics; this effect isn’t occurring in a vacuum, and the particles’ interactions with the fluid itself also result in tiny forces. The integrated effect of these forces across the entire system nets to zero, in compliance with the concept of equal and opposite reactions.

Golestanian firmly identifies as a theoretical physicist, but that doesn’t mean he is uninterested in the practical implications of his work. He says that the models the team has developed can apply to a multitude of situations, just as the equations for projectile motion apply equally to a shot put or a volleyball. “This is the beauty of theoretical physics,” he says. “When you build a theoretical framework to explain a phenomenon, there could be many realizations of the type of system that would follow those rules.”

Many bacteria use chemical gradients and catalysis to signal each other and hunt for food, for example. At the same time, Golestanian expects the same equations to (at least partially) describe how subcellular structures can form from individual enzymes, or how man-made Janus colloids aggregate in a Petri dish. Although he intends to stick with theory, he is excited to see where experimentalists take his framework. “The ultimate reward is when something comes out of the theoretical investigation which can later on be found in labs,” he says. And who knows? Maybe one of them will help answer Golestanian’s questions.

–Eleanor Hook

Eleanor Hook is a freelance science writer based in Chapel Hill, NC. She contributes regularly to Physics Buzz, where she writes about everything from dead fish to lasers in space.


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