You wouldn’t think molecules could be left- or right-handed – they’re too tiny to have limbs, after all. However, they absolutely can be, and it’s all to do with a property called chirality.
Chirality is defined by the International Union of Pure and Applied Chemistry (IUPAC) as “the geometric property of a rigid object (or spatial arrangement of points or atoms) of being non-superposable on its mirror image,” with objects without this property being achiral.
If you have two hands, you can look at an example of chirality right now, as they are mirror images of each other and can’t be superimposed. Try putting a right-handed glove on your left hand or vice versa if you want a demonstration (or read up on the love life of Jeremy the lefty snail). In fact, the word “chiral” comes from the Greek word for “hand”.
So how does this property arise? When two compounds of the same chemical formula have different arrangements of atoms, they are called isomers. Structural isomers are joined together in different ways, and stereoisomers have the same connectivity but different arrangements of atoms in space. Chiral molecules come in a type of stereoisomer called an enantiomer, where the two forms are non-superimposable mirror images.
Atoms that can bond to four others – carbon being the most common example – can be the core of a chiral molecule if the bonds are to four different groups, known as a chiral center or a stereogenic center. Bonds to two identical groups would give the molecule a plane of symmetry, making it achiral.
One fascinating property of chiral molecules is how they rotate plane-polarized light. Light is an electromagnetic wave that oscillates up and down perpendicular to its direction of movement. Normally, the wave can oscillate in any orientation, but polarized light only wiggles in one plane. In chiral molecules, one enantiomer rotates it in one direction, while the other enantiomer rotates it in the other.
This rotation of polarized light leads to one of the multiple naming systems for these isomers, originating from the optical properties of the sugar glyceraldehyde. Dextrorotatory, d, or (+), from the Latin “dexter” meaning “right”, rotates the light clockwise; and levorotatory, l, or (-), from the Latin “laevus” meaning “left”, rotates it counterclockwise.
This video from the Royal Society of Chemistry demonstrates this cool property using different sugars.
If you’re reading this thinking it’s just all chemistry jargon that has no application to real life, think again. Firstly, you can sometimes smell or taste the difference – for example, carvone, a terpene found in some essential oils, smells like spearmint in one form and caraway seeds in another.
Of the 20 amino acids that make up the proteins in our bodies, all are chiral apart from one: glycine, with a central carbon atom bonded to two hydrogen atoms, making it achiral. In fact, all these building blocks of life are found naturally in the L configuration, or "left-handed" – and this has big implications, for example, affecting how enzymes bind to their substrates.
One hypothesis as to why this is the case lies in star-forming clouds, where one of the hydrogens in glycine could have been displaced by the heavier hydrogen isotope deuterium, making it chiral.
However, in 2023, researchers published a paper theorizing that it could be down to the weak nuclear force driving the generation of a specific enantiomer of glyceraldehyde, leading to a preference for one chiral form over time.
“The reason why many of the key molecules of life only have one preferred handedness is a bit of a mystery,” study co-author James Cowan, a distinguished university professor emeritus in chemistry and biochemistry at Ohio State, explained in a statement. “As to how it came about, the process must reflect something very special about how early chemistry developed a preferred form of nucleic acids and proteins.”
One area where chirality is a key consideration is drug development. Take thalidomide as an example: the drug is chiral, and as the American Chemical Society explains, “Under biological conditions, the isomers interconvert, so separating the isomers before use is ineffective.” The issue is, whilst one enantiomer has the intended effect as a sedative, the other is teratogenic, causing the fetal abnormalities the drug is most known for today.
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