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

It was found by researchers at Penn State and the University of Maine that molecules interact non-reciprocally in the absence of outside stimuli.

Reciprocal fundamental forces include gravity and electromagnetism, which describe the attraction or repulsion of two things. However, interactions do not seem to follow this reciprocal pattern in our day-to-day experiences.

For instance, even if a predator is drawn to its victim, the latter usually runs away from it. These kinds of non-reciprocal interactions are necessary for sophisticated behavior found in living things. Hydrodynamic forces or other external forces have been proposed as the mechanism of non-reciprocal connections for microscopic systems like bacteria. Previously, it was believed that analogous forces might also account for interactions between single molecules.

Theoretical physicist R. Dean Astumian of the University of Maine and colleagues Ayusman Sen and Niladri Sekhar Mandal of Penn State have reported a novel method in which single molecules can interact non-reciprocally without hydrodynamic consequences. Their work was published in the journal Chem.

An enzyme is a biological example of a chemical catalyst. This process evokes the local gradients of reactants and products owing to the reactions promoted by each catalyst. It is conceivable to create a scenario where one molecule repels but attracts another since the catalyst's reaction to the gradient relies on its characteristics.

During their debate, the authors had their "Eureka moment" when they discovered that the kinetic asymmetry, a characteristic of all catalysts, governs the direction of the reaction to a concentration gradient. Kinetic asymmetry can change and adapt since it is an intrinsic feature of the enzyme.

Kinetic asymmetry facilitates non-reciprocal interactions that are essential for molecular interactions and may have been a major factor in the processes leading from basic matter to complex matter.

Other scholars have done a great deal of prior study on the consequences of non-reciprocal interactions. These initiatives have been crucial to the growth of the discipline known as "active matter." Ad hoc forces were used to introduce non-reciprocal interactions in this previous study.

However, the research reported by Mandal, Sen, and Astumian outlines a fundamental chemical process by which single-molecule interactions of this kind might occur. The present study is an extension of previous research conducted by the same scientists, which demonstrated the utilization of energy from the process it catalyzed to drive directional motion in a concentration gradient by a single catalyst molecule.

The design of synthetic molecular motors and pumps has taken into account the kinetic asymmetry that is present in defining the non-reciprocal interactions between various catalysts and has been demonstrated to be significant for the directionality of biomolecular machines.

Astumian, Sen, and Mandal are working together to uncover the principles of organization behind the ad hoc relationships between various catalysts that might have generated the first metabolic structures, which in turn led to the emergence of life.

"We're at the very beginning stages of this work, but I see understanding kinetic asymmetry as a possible opportunity for understanding how life evolved from simple molecules," Astumian stated. "Not only can it provide insight into complexification of matter, kinetic asymmetry can also be used in the design of molecular machines and associated technologies."