Are we closer to understanding the origins of biological homochirality?

“When I give talks to a general audience, I say, ‘Turn to your neighbor and shake hands,’ and you each put out your right hand. Then one of you put out your left hand, and that feels very different—it’s a different signal, and that’s essentially how chiral molecules work in the body,” says chemist Donna Blackmond, who studies chemical aspects of the origins of life at Scripps Research in California.

Given the complex organic molecules involved in biology, chirality was always likely to play a role. But when so much of biology relies on having the right 3D geometry, only the correct handedness will fit. The amino acids that make up proteins almost universally exist in the l form, whereas the ribose and deoxyribose sugars that make up the backbone of RNA and DNA are the opposite d form, with the homochirality crucial to produce a helical shape.

Today, biology can make homochiral molecules using enzymes designed with asymmetric 3D pockets that catalyse the production of only one of the possible mirror images of a molecule. But before proteins existed, how did a source of homochiral molecules form?

Explaining homochirality is still one of the big puzzles in understanding the chemical origins of life.

Could the chirality of life be seeded from space?

Without homochirality, biology—or at least our biology—could not have developed. We know that fact from experiments carried out in 1984 by chemist Gerald Joyce from the Salk Institute for Biological Studies. He showed that using chemically activated nucleotides, rather than enzymes, you can extend an RNA strand only if the same-handed enantiomer is present. If a racemic mixture of nucleotides is used instead, and an opposite-handed monomer is incorporated, it will block further extension, a mechanism he coined enantiomeric cross inhibition (Nature 1984, DOI: 10.1038/310602a0).

Joyce’s experiments showed that the presence of molecules of the other handedness can stop RNA polymers from forming. So without a homochiral source of starting materials, you cannot build RNA, which many think was the first self-replicating and information-carrying molecule. “Life is only really going to get going if [homochirality has] been dealt with beforehand; you want a stereochemically clean start,” says organic chemist John Sutherland from the MRC Laboratory of Molecular Biology.

Researchers have suggested that the origin of homochirality could be chemical reactions in which one reaction product acts as a catalyst for its own formation, a situation known as autocatalysis. In this case, the production of one enantiomer would be faster than the other.

One such reaction was discovered by chemist Kenso Soai from Tokyo University of Science in 1995. He reacted an organozinc reagent with pyrimidine-5-carbaldehyde to produce a chiral alcohol, which then catalyzes the production of 99.75% of one enantiomer (Nature, DOI: 10.1038/378767a0). “It’s a beautiful autocatalytic reaction,” Blackmond says, “but it’s completely prebiotically irrelevant, because it doesn’t work in water.”

An alternative theory comes from the Murchison meteorite, which fell in Australia in 1969. Researchers measured a 3–15% excess of several L-amino acids not found in proteins (Science 1997, DOI: 10.1126/science.275.5302.951) and suggested that this excess was caused by circularly polarized ultraviolet light in space, which will interact differently with both hands and cause one enantiomer to be preferably destroyed.

More recently this theory has been questioned, as several meteorite samples collected in space have failed to show any chiral imbalances in detected amino acids. Researchers have started to suspect that the imbalances detected in earlier samples may have been due to contamination from amino acids on earth.

Further support for the contamination theory came from a meteorite that fell on a driveway in Winchcombe, England, in 2021, which was collected within hours and showed no enantiomeric excess (Meteorit. Planet. Sci. 2023, DOI: 10.1111/maps.13936).

So the origins of homochirality have remained a puzzle, but some researchers now think they have an answer.

The start of solving symmetry breaking

Blackmond says the community of scientists studying the origins of life has gone through a fertile period in the last decade. For example, through a Simons Foundation initiative, researchers working on the origin-of-life puzzle “used to meet three times a year and just talk,” she says. “We started out with a list of questions, and we got pretty far on quite a few of them.” Now plausible routes exist to synthesizing RNA building blocks (Nature 2009, DOI: 10.1038/nature08013) as well as to assembling proteins and lipids (Nat. Chem. 2015, DOI: 10.1038/nchem.2202).

Homochirality is not an easy problem to solve. “There’s two parts,” Blackmond says. First you need a mechanism that creates an imbalance in one enantiomer over the other, known as symmetry breaking, and then you need a way to amplify that difference.

The first step might be a random occurrence. “You can never totally rule out stochastics,” says chemist and origin-of-life researcher Matthew W. Powner from University College London, though he adds that the research community has never found randomness to be a satisfying explanation for life’s homochiral start.

To suggest plausible mechanisms for that first step, Blackmond looked for autocatalytic reactions similar to Soai’s discovery but based on chemistry that would have been possible on the prebiotic Earth. She became interested in a reaction published by Powner to make the sulfur-containing amino acid cysteine.

Cysteine is the one biologically relevant amino acid that cannot be made via an aminonitrile—the assumed prebiotic route for all other amino acids—because its nitrile is not stable. Powner’s method to make cysteine started with the amino acid serine. He first acylated the amine as a chemical protection step to prevent degradation of the cysteine nitrile before further conversion through reacting with hydrogen sulfide to give N-acetylcysteine (Science 2020, DOI: 10.1126/science.abd5680). This method echoes biology itself, which produces cysteine as a secondary product of serine.

When Powner came up with his synthetic route, he also made an additional discovery: the cysteine he produced catalyzed the reaction that joins amino acids together to form peptides, which could suggest how peptide chains were formed before enzymes. Forming peptide bonds in water is thermodynamically unfavorable, but in this case, the thiol group of N-acetylcysteine attacks the nitrile of the α-amidonitrile to form a highly reactive thioimidate intermediate, which is more susceptible to attack by an amine—driving peptide bond formation in water.

Essentially, small amounts of cysteine-containing peptide catalyze the formation of more peptides. “It’s got that feel of autocatalysis,” Powner says. But the one catch is that the reaction favors coupling amino acids with different handednesses, which at first glance seems unlikely to lead to our homochiral world.

When Blackmond learned of this enantiomeric pairing preference, she started to think it might be able to help the homochirality cause, using the principle of kinetic resolution, where two competing pathways are differentiated by their relative reaction rates—in this case, one enantiomer reacting faster than another in the presence of a chiral catalyst.

Starting with a small imbalance toward the L enantiomer, she measured the handedness of the peptide products using nuclear magnetic resonance spectroscopy and found, as expected, that most reactions were between differently handed molecules, leading to products made up of both an L- D-amino acid. That outcome, she says, isn’t a problem, because those peptides are less soluble and precipitate out.

“Every time you take an l and d and make an LD complex, that’s out of the picture, so it actually amplifies the LLs that are left, over the DDs,” Blackmond explains. In other words, what’s left in solution will now have an even higher proportion of the L enantiomer than the starting imbalance. Even though the reaction with the same hand is slower, some product will form, and because of the increased imbalance, what forms is more likely to be LL than DD.

Her experiments showed that with a 20% excess of the L monomer and after removing several precipitated products (including a crystal containing both homochiral peptides), the imbalance in solution reached 80% LL over DD. It seems counterintuitive, but in this case, the monomers staying in solution long enough for amplification to occur provided an advantage.

Could this mechanism explain the emergence of homochiral peptides? Blackmond says it comes closer to a plausible autocatalytic mechanism than previous examples, but she knows it doesn’t solve the symmetry-breaking problem: “You still have to have a chiral source to get any kind of differentiation,” she says.

Powner thinks Blackmond’s results are a creative solution to the problem of making homochiral peptides when the reaction isn’t favored. “I think the term she used once was making lemonade out of lemons,” he jokes. But Blackmond herself says she isn’t betting on her results being the answer to biological homochirality.

The last decade of continued debate with her colleagues has made Blackmond look to a different set of starting molecules. “Maybe we don’t need to find a way to make the amino acids,” she says. “If I had to make a bet, I would bet on nucleic acids first.”

Were nucleic acids the first chiral molecules?

Nucleic acids have also been the starting point for Sutherland. Sutherland’s group was first to come up with a prebiotically feasible synthetic route to RNA nucleotides from basic chemical building blocks, eventually as small as hydrogen cyanide.

But a link also exists between the chirality of RNA polymers and amino acids, which could explain how L-amino acids were selected in biology. “If we have enantiomerically pure RNA and racemic amino acids, the chirality of the RNA can be passed on to the amino acids,” Sutherland says. So if homochirality can be propagated from nucleic acids through to peptides, one needs to explain only how to get a source of homochiral nucleotides.

California Institute of Technology physicist S. Furkan Ozturk agrees that starting with ribonucleotides makes more sense, given that the evidence for chiral enrichment of amino acids on meteorites is now much less certain. But rather than looking for a chemical answer for the chirality of the first ribonucleotides, Ozturk thinks a magnetic factor may be at play.

In 1999 Ron Naaman at the Weizmann Institute of Science showed that the chirality of a chemical compound is coupled to its electron spin, so each enantiomer will preferentially transmit electrons of up or down spin (Science, DOI: 10.1126/science.283.5403.814). This effect is called the chiral-induced spin selectivity (CISS) effect, and conversely, Ozturk explains, it means that a ferromagnetic surface will preferentially attract one hand of a molecule over the other.

According to Ozturk, a chiral molecule approaching a ferromagnetic surface will experience a local magnetic field, which interacts with its electrons as well as transiently creating a charge dipole, which generates spin polarization. The enantiomer with its spin aligned to the magnetic surface is adsorbed onto that surface more easily.

At a meeting that included Sutherland, Ozturk, and Ozturk’s then PhD supervisor Dimitar Sasselov, the researchers decided to test if this effect could explain the origins of homochirality. To do the experiment, they homed in on the molecule ribo-aminooxazoline (RAO), which is one of the intermediates in Sutherland’s synthesis of RNA nucleotides. The big advantage is it’s relatively insoluble and forms a crystal with only one enantiomer of the molecule, a type of crystal known as a conglomerate.

Starting with a racemic solution of RAO on a thin (200 nm) film of magnetized magnetite for up to a day, Ozturk found that the crystals that formed had up to a 60% excess of one enantiomer. When the magnetic field was reversed, so was the hand forming in excess. A second crystallization produced completely homochiral crystals of D-RAO in just two steps (Sci. Adv. 2023, DOI: 10.1126/sciadv.adg8274).

Ozturk says what is exciting about the result is that the process both breaks symmetry, by selecting which hand nucleates on the surface first, and then amplifies the effect because of the crystallization as a conglomerate. “RAO is the only conglomerate-forming compound in the line of synthesis of RNA, so it’s the only compound that can do this,” he says.

For Sutherland, the idea is compelling because it fits perfectly with developing geochemical scenarios that envisage these reactions happening in natural, shallow evaporative lakes containing magnetite sediments, where wet-dry cycles could dissolve and recrystallize RAO. “There are lots of places on Earth, even now, where rivers or bodies of water which contain high levels of solutes evaporate and deposit those solutes as crystals or solids. So it’s not invoking something which is crazy or unheard of,” he says.

The size of the magnetic field used in the RAO experiments was stronger than the magnetism that would be created in rocks from the Earth’s field, so whether the experiments represent a real-world scenario is in question. But Ozturk is now extending his work to real rock surfaces with iron-containing inclusions, like magnetite or greigite, which might better mimic the conditions of early Earth. “We want to see the effect working and how strongly it is working on those natural systems,” he says.

Electron spin and homochirality

Blackmond says the CISS theory caused excitement in the field and “brought chirality back into the news cycle.” She jokes that the introduction of this “esoteric” physical principle “sounds sexy.”

But Sutherland sees this theory as important, not just because it provides a possible solution to the homochirality puzzle but also because it fits with his other work on RAO. “The fact that you could slot [the CISS explanation of homochirality] into the puzzle means that the bits of the puzzle that you had in place before are probably also correct,” he says. As well as the prebiotically plausible synthetic routes to pyrimidine nucleotides of RNA and DNA via RAO, Sutherland says his group members now have results, not yet published, that suggest they can also make the two purine nucleotides via RAO. Doing so would mean that the chiral control of RAO could control the chirality of all four nucleic acids.

Sutherland thinks CISS “has all the hallmarks of being something which is going to grow in acceptance rather than diminish.”

Yet wider uncertainties around the CISS effect still exist. “At the very fundamental level, there’s still a debate about how and why it works,” says Matthew Fuchter, a chemist working on technological applications of chiral materials at the University of Oxford.

Predictions made by the theoretical models based on spin-orbit coupling consistently fail to match the larger scale of measured effects. As yet, no universally accepted theory capable of accounting for the results exists.

In addition to this uncertainty, Fuchter says efforts to exploit the effect have not always been successful. For example, the pharmaceutical industry has sponsored research into using CISS for chiral separations of enantiomers from racemic mixtures, but the methods tested have not reliably worked.

Nevertheless, Fuchter says a magnetic substrate could plausibly cause enrichment in one hand of a molecule, although “I don’t see it as a panacea.” For now, he says, too many uncertainties exist to be confident about the mechanism.

No time machine can take us back to find out, but the origin-of-life scenarios currently emerging are certainly more plausible than ever, which is why Blackmond says the 2023 end of the very stimulating Simons Foundation collaboration, which led to so many advances, is so frustrating.

Ozturk says he is determined to keep going. His ultimate dream is to show that going from small-molecule starting materials in a natural environment to an excess of right-handed ribonucleotides is experimentally possible. That excess could have given rise to the homochiral biology we see today.

There may, of course, be dead ends and more steps to discover along the way. “We don’t get everything right at the beginning,” Ozturk says. “We get pieces right, and then we start seeing the big picture a little bit better in the end.”

Rachel Brazil is a freelance writer based in London.