Some molecules tend to exist in two different forms: “right” or “left”, related, just as our hands are, by symmetry, through a plane of reflection or mirror. The crystals of these substances can be of just one form or a mix of both. Both these molecules and the crystals made up of them are called “chirals”. The biological activity of the molecules and the physical properties of the crystals change drastically with chirality. For example, limonene is a chiral molecule that when “left-handed” smells of orange and when “right-handed” smells of lemon.
Did you know that all of the amino acids that form proteins are “left-handed” while the sugars of the nucleic acids are “right-handed”? Did you know that crystallization is one of the most effective methods for separating the right and left forms? And do you know why the properties of the two chiral compounds are so different?
The two hands on this poster, like your two hands, are equal but different, as you well know. One is your right hand and the other your left. You know they are different because, among other things, you can’t put a left-hand glove on your right hand, and vice versa.
There are also spiral staircases that are right-handed, in which the spiral rises in a clockwise direction, and left-handed, in which the spiral rises counter-clockwise. We call this property of one same object having two configurations, left and right, chirality, and these objects are called chiral. The example of hands is perfect. In fact, the word chirality comes from the Greek kheir, meaning hand.
Like hands, there are molecules that are chiral. For example, the molecule of this amino acid. And there are also crystals that are chiral, for example, tartaric acid. (Left picture, Yassine Mrabet).
Their role in chemistry and biochemistry is extraordinary. Let’s look at an example.
You will have noted that in our poster we have used oranges and lemons as background, with a molecular structure superimposed. This structure is that of limonene, a chiral molecule that in its left-handed form smells of orange and in its right-handed form of lemon. How can this be? Because of our olfactory receptors, which are also chiral: those that smell orange recognise the left-handed limonene and those that smell lemon recognise the right-handed limonene. How do they do it? In the same way that you can shake hands without looking with someone who offers you their right-hand and not with someone who offers you their left (for-left-handers, the other way around). By the way, both of the two chiral structures are called enantiomers.
Chirality seems to be an essential property for life. The capacity that the chemistry created by life has for discriminating between the different enantiomers is essential for living systems. The amino acids that form proteins are chiral but what is curious is that all of our amino acids are left-handed. Even more curious, the sugars that form nucleic acids are all right-handed. In fact, one of the most difficult questions to answer about prebiotic chemistry – the chemical route that eventually led to the production of the macromolecules of life – is the way in which they selected only molecules of one type of chirality.
Many medicines are formed by molecules that are chiral, given that the receptors they couple to are as well. Their effect is therefore very specific depending on whether the medicine is a left- or right-handed chiral molecule. Thus it is very important to know how to crystallize only the appropriate enantiomer, because the other would not be efficacious because it would not couple with the correct receptor. And there are tragic examples of this, as in the case of thalidomide, which you can learn about in detail here. This video may also interest you:
The case of thalidomide is a dramatic example of the importance of chirality. This drug was commercialised in 1956 throughout the world as a sedative, which became very popular as it could even be taken in large doses. It was used particularly by pregnant women to combat insomnia, anxiety and morning sickness. However, at the beginning of the sixties it was discovered that a growing number of babies were being born with congenital (teratogenic) deformities to mothers who had taken thalidomide during the first three months of pregnancy. This was because the commercialised drug was a racemic form – in other words, it contained both right-handed and left-handed molecules. Left-handed thalidomide really does have a sedative effect, but the right-handed form is teratogenic – it created malformations in foetuses. This is an unfortunately dramatic way of demonstrating the importance of crystallography in the production of pharmaceuticals.
The sculpture “Hands and Molecule” was designed by David Barnes and commissioned by Pfizer. The sculpture was presented in 2000 to celebrate the opening of the National Cycle Network (Route 15) in Thanet. This sculpture is situated on the West Cliff, Ramsgate, overlooking Pegwell Bay, at the eastern end of the English Channel.