Homochirality refers to the uniformity of chirality, or handedness. An object is chiral if it cannot be matched exactly with its mirror image. For example, a person’s left and right hands are mirror images of each other but cannot be made to match exactly, so they are chiral. In chemistry, chirality is a geometric feature of certain compounds and ions. These compounds can exist in two different forms, called enantiomers, often described as left-handed and right-handed versions of a compound (labeled L- and D-, respectively, based on how they rotate plane-polarized light). The term homochiral describes substances that contain only one type of enantiomer.
Enantiomers have the same chemical properties in environments that are not chiral, so non-living chemical processes usually create racemic mixtures, which are mixtures with equal amounts of L- and D-isomers. However, many substances made by living organisms are homochiral. For example, 19 of the 20 amino acids used to build proteins in living things are left-handed, except for glycine, which is not chiral. Biological sugars are also right-handed.
Scientists have proposed several theories about the role of homochirality in nature. It may help store information and lower energy barriers when forming large, organized molecules. Experiments show that amino acids form more large groups from enantiopure samples (samples with only one type of enantiomer) than from racemic mixtures. Small amounts of the opposite enantiomer can also slow down processes like RNA replication and chain growth, which are important for life.
Since homochirality is common in living organisms, a major question in research on the origins of life is how homochirality developed from racemic mixtures of simple chemical building blocks. Scientists have suggested several possible mechanisms. Some models propose three steps: a process that breaks mirror symmetry to create a slight imbalance in enantiomers (called enantiomeric excess or ee), a process that increases this imbalance to achieve full homochirality (ee=100%), and a process that spreads chirality from one group of molecules to another. Another important factor is whether these processes could realistically occur under early Earth conditions using materials available at the time.
History of the term
In 1904, Lord Kelvin first discussed homochirality, a concept he mentioned in his 1884 Baltimore Lecture. He explained that homochirality describes a relationship between two molecules, meaning they share the same chirality. The term "homochiral" has been used similarly to "enantiomerically pure," which refers to a mixture containing only one type of mirror-image molecule. Some scientific journals allow this term, though they do not encourage its use. In these journals, "homochiral" means a process or system favors one optical isomer over its mirror-image counterpart.
In biology
Homochirality is a common feature in living things, found in the basic parts of large molecules like DNA and proteins. Amino acids, which build proteins and enzymes, are mostly left-handed. However, some right-handed amino acids in proteins come from changes that happen after the protein is made. Sugars like ribose and deoxyribose, which are parts of RNA and DNA, are all right-handed. Other molecules inside cells also show homochirality. For example, malate and isocitrate, which are part of the citric acid cycle, are left-handed and right-handed, respectively. In modern biology, enzymes help create this same-handedness in molecules, including hormones, toxins, and flavors.
Living things can tell the difference between molecules with different handedness. This affects how the body reacts, such as how things smell or taste. Carvone, a chemical in essential oils, smells like mint when it is left-handed and like caraway when it is right-handed. Limonene, another chemical, tastes like citrus when right-handed and like pine when left-handed.
Same-handedness also influences how drugs work. Thalidomide helps treat morning sickness when it is left-handed, but it causes birth defects when it is right-handed. Even if only the left-handed version is given, some of it can change into the right-handed form inside the body. Many medicines are sold as mixtures of both forms or as pure forms of one handedness. Pure forms can be more expensive to make, depending on how they are produced.
Same-handedness is also seen in larger structures. Snail shells can twist to the left or right, but most species strongly favor one direction. For example, in the edible snail Helix pomatia, only one out of 20,000 has a left-twisting shell. Plants can also have a preferred handedness in how they grow, and even cows chew food with a slight preference for one direction over the other.
Origins of biomolecular homochirality
Theories about how life's molecules became homochiral (having a single handedness, like left or right) can be grouped into two types: deterministic or based on chance. If a specific cause, such as a chiral field or influence, leads to mirror symmetry breaking, the theory is deterministic. If no specific cause is involved and randomness is the reason, the theory is based on chance.
Deterministic theories are divided into two groups. If the initial chiral influence happened in a specific place or time, the theory is called local deterministic. If the chiral influence was always present when chiral selection occurred, it is called universal deterministic. These groups can overlap. Even if an external chiral influence caused the initial imbalance, the final outcome might still be random because the influence has a mirror counterpart elsewhere.
In deterministic theories, an external chiral field or influence creates an imbalance between mirror-image molecules. The handedness of biomolecules depends on this influence. Examples of deterministic mechanisms include physical laws like the electroweak interaction (through cosmic rays) and environments like circularly polarized light (CPL), quartz crystals, Earth's rotation, beta-radiolysis, or the magnetochiral effect. For example, shortwave circularly polarized light can cause mirror-image molecules to absorb light differently, creating a bias.
Parity is a symmetry property that holds for strong, electromagnetic, and gravitational interactions. However, experiments with weak forces, like those between subatomic particles, show that parity is not conserved. This could create tiny energy differences between mirror-image molecules, favoring one handedness. However, no experiment has yet shown a clear link between molecular chirality and these energy differences.
Some studies of organic molecules in space, like meteorites and asteroids, found small imbalances in certain amino acids and sugars. These findings suggest that mirror-image bias might have come to Earth from space. The similarity between these imbalances and those in living organisms supports this idea. Experiments also show possible ways to create such imbalances in space materials.
It was proposed that circularly polarized light from interstellar dust particles could create mirror-image imbalances in space. Magnetic fields in space might align dust particles to produce this light. Another idea, the Vester-Ulbricht hypothesis, suggests that physical processes like beta decay (related to parity violation) might cause slight differences in the lifetimes of certain molecules.
Asteroids Bennu and Ryugu have no chiral bias in their amino acids. The Murchinson meteorite contains only racemic (equal) amino acids.
Chance theories assume that "absolute asymmetric synthesis," where mirror-image molecules form without chiral reagents or catalysts, is unavoidable due to randomness.
Most symmetry-breaking mechanisms focus on amplifying a small initial imbalance. The most likely way this happens is through asymmetric autocatalysis. In autocatalysis, the product of a reaction speeds up the reaction itself. In asymmetric autocatalysis, a chiral molecule acts as its own catalyst. A small initial imbalance, like from polarized light, allows one mirror-image form to dominate.
In 1953, Charles Frank proposed a model showing how homochirality could arise from autocatalysis. His model involves two mirror-image forms (L and D) of a molecule being produced from an achiral molecule A. These forms also react with each other in a process called mutual antagonism, reducing each other's concentration. The model shows that even a tiny initial imbalance leads to a complete homochiral state over time.
The ratio of L to D molecules grows rapidly if one is more abundant at the start. If the initial amounts of L and D are equal, the system is unstable, and the mixture eventually becomes completely homochiral. This instability depends on the mutual antagonism reaction.
The enantiomeric excess (ee) is a measure of the imbalance between mirror-image molecules. Calculating how ee changes over time shows that the racemic state (ee = 0) is unstable. Most starting conditions lead to a homochiral state.
Autocatalysis alone does not create homochirality, but the mutual antagonism between mirror-image forms is needed for the instability. Without this, the mechanism for symmetry breaking would be different.
A small amount of one mirror-image form at the start of a reaction can lead to a large imbalance in the product. For example, the Soai reaction is autocatalytic. If one mirror-image product is already present, it can speed up the production of more of the same form.
In the proline-catalyzed reaction of propionaldehyde with nitrosobenzene, a slight imbalance in the catalyst leads to a large imbalance in the product.
Mass spectrometry shows clusters of eight serine molecules. However, no evidence supports these clusters forming under non-ionizing conditions. Fractional sublimation of a 10% enantioenriched leucine sample…
Optical resolution in racemic amino acids
No theory explains why L-amino acids are so common. For example, take alanine, which has a small methyl group, and phenylalanine, which has a larger benzyl group. A simple question is: In what way does L-alanine resemble L-phenylalanine more than D-phenylalanine? What mechanism causes all L-amino acids to be chosen, because it might be possible that alanine was L and phenylalanine was D.