Molecules exhibit non-reciprocal interactions without external forces, new study finds

Researchers at the University of Maine and Penn State have discovered that molecules undergo nonreciprocal interactions without external forces.

Fundamental forces such as gravity and electromagnetism are reciprocal, in which two objects are attracted to each other or repelled by each other. However, in our daily experience, interactions do not seem to follow this reciprocal law.

For example, a predator is attracted to a prey, but the prey tends to flee the predator. Such nonreciprocal interactions are essential to the complex behaviors associated with living organisms. For microscopic systems such as bacteria, the mechanism of nonreciprocal interactions has been explained by hydrodynamic or other external forces, and it was previously thought that similar types of forces could explain interactions between single molecules.

In a work published in ChemistryUMaine theoretical physicist R. Dean Astumian and collaborators Ayusman Sen and Niladri Sekhar Mandal of Penn State have published a different mechanism by which single molecules can interact nonreciprocally without hydrodynamic effects.

This mechanism involves local gradients of reactants and products due to reactions facilitated by each chemical catalyst, a biological example of which is an enzyme. Since a catalyst’s response to gradient depends on the properties of the catalyst, it is possible to have a situation where one molecule is repelled but attracts another molecule.

The authors’ “Eureka moment” occurred when, during their discussion, they realized that a property of each catalyst known as kinetic asymmetry controls the direction of response to a concentration gradient. Since kinetic asymmetry is a property of the enzyme itself, it can undergo evolution and adaptation.

The nonreciprocal interactions enabled by kinetic asymmetry also play a crucial role in allowing molecules to interact with each other and may have played an essential role in the processes by which simple matter becomes complex.

Much previous work has been done by other researchers on what happens when nonreciprocal interactions occur. These efforts played a central role in the development of a field known as “active matter.” In this previous work, non-reciprocal interactions were introduced by incorporating ad hoc forces.

The research described by Mandal, Sen and Astumian, however, describes a basic molecular mechanism by which such interactions can arise between single molecules. This research builds on previous work in which the same authors showed how a single catalyst molecule could use the energy of the reaction it catalyzed to undergo directional movement down a concentration gradient.

The kinetic asymmetry that determines nonreciprocal interactions between different catalysts has also been shown to be important for the directionality of biomolecular machines and has been incorporated into the design of synthetic molecular motors and pumps.

The collaboration between Astumian, Sen and Mandal aims to reveal the organizational principles behind the loose associations of different catalysts that may have formed the first metabolic structures that ultimately led to the evolution of life.

“We are only in the very early stages of this work, but I see understanding kinetic asymmetry as a possible opportunity to understand how life evolved from simple molecules,” says Astumian. “Not only can it provide insight into the complexification of matter, but kinetic asymmetry can also be used in the design of molecular machines and related technologies.”