The way chemical systems adapt to an energy source can lead to spontaneous chiral symmetry breaking, according to new findings from researchers in South Korea. The discovery could help explain the origins of biological homochirality, the phenomenon in which many biomolecules exist solely as one enantiomer in nature.
Compounds such as amino acids and sugars are essential for life and form the building blocks of peptides, enzymes, and nucleic acids. But in nature, amino acids almost always occur in their left-handed chiral form, whereas sugars are exclusively right-handed. This chirality is passed on to many other biomolecules, but its origins on early Earth remain an enigma.
Various theories have suggested that the origin of the homochirality of life could lie in polarized radiation, temperature gradients, kinetic resolution, or simply the direction in which the reaction components are stirred.
William Piñeros and Tsvi Tlusty of the Ulsan Institute of Basic Sciences and the Ulsan National Institute of Science and Technology wanted to know if the way energy is exchanged and consumed could induce asymmetry in a complex chemical system.
“This question is very natural when considering an engineering system like a car engine, where we understand that the configuration of the system, the arrangement of the shafts and pistons, affects how efficiently it can harness an energy source” Tlusty says. “But the same ideas also apply to random chemical networks: certain configurations of these networks are better suited than others to the energy source, and thus can exploit it much more efficiently, leading to matching configurations and so on.” successively in a reinforcing loop”.
To illustrate this idea, Tlusty gives the example of a photoinduced chemical reaction whose products dissolve light-obscuring products in a turbid pond. As the reaction proceeds, the intensity of the light in the pond will increase and thus the rate of the reaction will speed up.
Piñeros and Tlusty developed a theoretical model to simulate whether this type of reinforcing loop could drive a chemical system towards single-enantiomeric products, without the product molecules themselves directly catalyzing the process.
“We observed that a random chemical network, starting from a completely symmetric state in which the number of left-handed and right-handed enantiomers of all molecules is the same, could suddenly change to an asymmetric state with a strong bias towards one of the enantiomers, “Importantly, this symmetry breaking requires strong environmental forcing but not autocatalysis, as previously assumed,” says Tlusty.
Tlusty highlights three broader impacts of the find. ‘First, asymmetric chemical synthesis of chiral molecules can be induced by adapting energy exchange,’ he says. “Second, in the context of early-life scenarios, homochirality could arise due to an energy-harvesting advantage over racemic competitors.” Finally, Tlusty points out that the symmetry-breaking mechanism is general and could therefore be applied to features of chemicals other than chirality.
“Complete breaking of chiral symmetry in certain classes of self-replicating biological molecules at the dawn of life requires out-of-equilibrium autocatalysis, but until now, the precise way in which external environmental drives are coupled to the nascent biological system has been a mystery. mystery.’ says Nigel Goldenfeld of the University of California, San Diego, an expert in living systems theory and nonequilibrium statistical physics.
Goldenfeld notes that an “unexpectedly cool discovery” of the study is that homochirality only arises when particular conditions that match the ambient energy source are met. “Until this adaptive feedback mechanism kicks in, homochirality won’t emerge in your model,” he says. “This is highly plausible and suggests that generic features of living matter arise spontaneously through rare fluctuations that then react in reaction networks to stabilize highly dissipative conditions that allow strong coupling with the environment.”
Piñeros and Tlusty’s results “add details and nuances” to previous research on homochirality “but now in a much more complex reaction network model,” says Goldenfeld. “The importance of all these theoretical studies is that they provide a basis for inferring that homochirality is a true biosignature, not an accident, and thus it is a valid way to infer the presence of life if homochirality and not simply enantiomeric excess it can be detected in planetary studies,’ he adds.