All life uses left-handed amino acids and right-handed sugars. Why life chose one mirror image over another remains biology's deepest unsolved mystery.
User:Basilicofresco, CC BY-SA 3.0, via Wikimedia Commons
A discovery in a wine barrel
In Paris in 1848, a young chemist named Louis Pasteur was investigating a substance called tartaric acid, a compound left behind during wine fermentation. He was puzzled by a problem posed by the respected physicist Jean-Baptiste Biot: two forms of tartrate salt looked identical and had the same chemical formula, yet one rotated plane-polarized light while the other did not.
Working with a pair of tweezers and a microscope, Pasteur began sorting the tiny salt crystals of sodium ammonium tartrate. He noticed that the crystals came in two distinct shapes, each a non-superimposable mirror image of the other, like a pair of hands. On their own, each crystal type was "optically active." When Pasteur dissolved the "right-handed" crystals in water, the solution rotated polarized light to the right. The "left-handed" crystals rotated light to the left. The original mixture was optically inactive because it contained a 50/50 mix of both types, canceling each other out.
Pasteur had demonstrated molecular chirality—the idea that molecules can exist in left- and right-handed forms, called enantiomers. He then made another observation: when a mold, Penicillium notatum, was grown in a solution of the racemic mixture, it consumed only the right-handed tartaric acid, leaving the left-handed form behind. This showed biological processes are stereospecific; they can distinguish between molecular mirror images. This was the first clue to a universal pattern that is found in all life.
Life's universal preference
The principle Pasteur found in Paris extends to the fundamental building blocks of all known organisms. The 20 common amino acids used to build proteins are, with very few exceptions, "left-handed" (L-isomers). Conversely, the sugars that form the backbone of DNA and RNA—deoxyribose and ribose—are exclusively "right-handed" (D-isomers). This preference for one mirror image over the other is called homochirality.
This molecular bias is absolute. A right-handed D-amino acid will not fit into the cellular machinery designed for its left-handed L-amino acid counterpart. Life depends entirely on this consistent single-handed structure. Yet, abiotic chemistry produces racemic mixtures—equal amounts of both left and right-handed molecules. So, how did life settle on this specific arrangement?
The origin of homochirality is an unanswered questions in biology. One hypothesis suggests that circularly polarized light from neutron stars or supernovae could have preferentially destroyed one enantiomer of the primordial molecules on the early Earth. Another theory points to an extraterrestrial origin. Analysis of the Murchison meteorite, which fell in Australia in 1969, revealed over 70 different amino acids. Several of these amino acids showed a slight excess of the left-handed form, suggesting that a bias for L-amino acids may have been delivered to Earth from space.
💡Fun Facts
The molecule carvone demonstrates how chirality affects smell. (S)-carvone is responsible for the scent of caraway, while its mirror image, (R)-carvone, smells of spearmint.
The drug thalidomide was a tragic lesson in chirality. Marketed in the 1950s, its (R)-enantiomer was an effective sedative, but its (S)-enantiomer was a teratogen that caused severe birth defects.
Limonene is another molecule whose smell depends on its handedness. (R)-limonene smells of oranges, while its mirror-image twin (S)-limonene smells of lemons.
While nearly all life uses L-amino acids, some bacteria incorporate D-amino acids into their cell walls, which helps them resist attack by enzymes that are specific to the L-form.
This is a conceptual POI tied to Louis Pasteur's discovery in Paris. To connect with the history, you can visit the Musée Pasteur. The museum is currently closed for renovation until November 2028. When open, it typically offers guided tours Monday to Friday.
Admission
The Musée Pasteur charges an admission fee.
Accessibility
The museum is located within the historic Pasteur Institute building, which includes multi-story elements like Pasteur's former apartment.
