Many of life’s molecules exist in two forms that are mirror images of each other, much like a pair of human hands. This property is called chirality. While these molecules are chemically identical, they are not superimposable. Homochirality describes how life on Earth exclusively uses one “handed” version of these molecules, a uniform preference that is a consistent chemical signature across all known organisms, from the simplest bacteria to the most complex animals.
The Handedness of Life’s Molecules
A strict rule governs the handedness of life’s molecular components. All proteins are constructed exclusively from left-handed (L) amino acids, while the sugar molecules that form the backbone of genetic material, DNA and RNA, are always in their right-handed (D) form. This consistency is a requirement for the proper function of biological machinery.
This uniformity is explained by the “lock and key” analogy. The specific three-dimensional shape of an enzyme or receptor only accommodates a molecule with the correct chirality. A “wrong-handed” molecule, like a right-handed amino acid, would not fit into machinery designed for its left-handed counterpart, making it useless or even disruptive.
Molecular consistency also allows for the formation of stable, complex structures. The exclusive use of L-amino acids enables proteins to fold into predictable shapes like alpha-helices. Similarly, the use of D-sugars allows DNA to form its stable double helix. Without this uniform handedness, these orderly structures would be impossible to maintain.
The Origin Mystery
How this molecular preference, or homochirality, began is a major unsolved question. In standard laboratory chemistry, reactions that produce chiral molecules almost always result in a 50/50 mixture of left- and right-handed forms, a state known as a racemic mixture. Breaking this symmetry to favor one hand over the other is a significant hurdle in theories about the origin of life, and researchers are exploring several competing hypotheses.
One hypothesis suggests an extraterrestrial influence, where life’s building blocks arrived with a pre-existing chiral imbalance. Meteorites like the Murchison, which fell in 1969, support this by showing a slight excess of the L-amino acids that life uses. This has led to theories that circularly polarized light from sources like neutron stars selectively destroyed one enantiomer in interstellar dust clouds, seeding the early Earth with an enriched supply of the other.
Another theory proposes terrestrial origins, where a small, random imbalance on early Earth was amplified over time. This could have occurred through autocatalytic reactions, where a molecule catalyzes its own creation. In such a system, a minor initial advantage for one enantiomer, perhaps by chance, could become magnified through repeated cycles, leading to the dominance of one handedness where life emerged.
Consequences in Medicine and Technology
The handedness of molecules has significant consequences in medicine. A well-known example is thalidomide, a drug from the late 1950s used for morning sickness. Thalidomide was sold as a racemic mixture, containing both its left- and right-handed forms.
While the right-handed (R)-enantiomer was an effective sedative, the left-handed (S)-enantiomer was a teratogen, causing severe birth defects like malformed limbs. A complicating factor is that the two forms can interconvert within the body. Administering only the “safe” R-enantiomer would not have prevented the harm, as it would have converted into the S-form.
The thalidomide disaster highlighted the importance of chirality in drug development. The synthesis of “enantiopure” drugs, containing only the desired enantiomer, is now a focus of the pharmaceutical industry to maximize effects and minimize side effects. Chirality is also relevant elsewhere; the two enantiomers of carvone have distinct smells. The (R)-form smells of spearmint, while the (S)-form smells of caraway, due to the chiral nature of our olfactory receptors.
A Potential Signature for Extraterrestrial Life
The homochirality of terrestrial life has led astrobiologists to consider it a potential biosignature for life beyond Earth. Detecting a significant enantiomeric excess, or a strong preference for one handedness, in organic molecules on another world would be strong evidence for biology. Such a discovery on Mars or an icy moon like Europa could point toward a past or present biological system.
This concept is actively shaping the design of scientific instruments for planetary exploration. For example, the Mars Organic Molecule Analyzer (MOMA) instrument on the European Space Agency’s Rosalind Franklin rover was designed to separate and identify the chirality of amino acids it might find in Martian soil. The discovery of a strong chiral preference would be a powerful piece of data in the quest to determine if life ever arose elsewhere in our solar system.