In the beginning, there were chiral molecules. RNA, DNA, proteins, and other biological molecules tend to be present as only either right- or left-handed. However, scientists are not sure how this homochirality emerged: why did one enantiomer of a molecule become the dominant one, over its mirror image? New research exploring a popular theory about the rise of homochirality suggests that the answer is not so simple as researchers had hoped.
In living systems today, sophisticated enzymes selectively produce only one chiral version of biomolecules such as D-sugars and L-amino acids. Researchers have long wondered how this preference came to be on early earth, before stereochemically picky enzymes existed. They reason that there must have been something about prebiotic reactions that could have created life’s homochiral preferences. For an answer, some have turned to energy levels. Based on theoretical calculations, scientists have long known that enantiomers can have slightly different energies, to the tune of mere pico- to femtojoules per mole. Still, researchers believed these differences might be enough to favor homochirality in early, prebiotic reactions. Some have postulated a complex reason why that might be. For one enantiomer to be formed over the other, they reasoned, the reaction that produces it would have to be autocatalytic, directing the products to have the same chirality, possibly through forming dimers or tetramers in solution.
But some have doubted whether tiny enantiomeric energy differences were enough to push the reaction towards one symmetry or the other. Now, through a series of kinetic experiments, Donna Blackmond and Neil Hawbaker of Scripps Research in California were able to discover the energy difference required for a chiral, autocatalytic reaction to preferentially form one chiral product over the other. Based on their results, they argue that the energetics can’t account for one chirality to really be favored over the other in prebiotic reactions (Nat. Chem. 2019, DOI: https://doi.org/10.1038/s41557-019-0321-y).
Only one documented reaction has the asymmetrical autocatalytic qualities required by the popular prebiotic chemical theory: the Soai reaction discovered in 1995 by Kenso Soai of the Tokyo University of Science. The alkylation of pyrimidine-5-carbaldehyde with diisopropylzinc creates a chiral pyrimidyl alcohol, whose presence directs further products to have the same chirality. The Soai reaction likely did not occur on early Earth, but the researchers used it as a model since it’s unique in having this autocatalytic quality. Blackmond and Hawbaker ran a series of kinetic experiments where they combined successively shrinking ratios of isotopically chiral alcohols. As a control, the researchers also ran the reactions with the achiral form of the alcohol. They then determined the kinetic profiles of the reactions and found the threshold value to break symmetry was very small, lower than they could have measured using direct methods. Blackmond and Hawbaker then calculated that the energy required to break symmetry and consistently form one isomer over the other is in the ballpark of 1.5 × 10–7 and 1.5 × 10–8 kJ/mol. This implies that the threshold for symmetry breaking would involve an excess of one enantiomer in a concentration around 20 mg of the compound in an Olympic sized swimming pool—a concentration so small that it is unlikely that prebiotic chemical reactions would have been given a big enough push towards one enantiomer or the other, Blackmond says.
Due to the kinetics of the Soai reaction, “it is hard to imagine the outcome would be any different for a prebiotic reaction network,” says Joseph Moran, a chemist at University of Strasbourg. “The work effectively rules out one of the popular—and dubious in my opinion—explanations for how life became homochiral,” he says.