Chirality: a Brief Introduction

by Franky Elsmore

Chirality can be simply explained by looking at your own hands. Despite the fact that your hands are mirror images of one another, it is impossible to superimpose them. No matter how you may rotate your hands, it is impossible to get them to perfectly line up. The same happens with molecules: if there is a carbon with four different groups attached to it, two enantiomers  can occur. Despite being chemically identical, the positions of the different R groups are arranged differently in space and much like your hands they cannot be superimposed. These two different versions of the same molecule are indistinguishable from each other until they are placed into a chiral environment, and then their differing arrangements in space determine how they interact with other chemicals in the environment. Another ‘handy’ analogy is to think of a pair of gloves resting on a table top, both the left and right gloves respond to the environment in the same way - this is the achiral environment. But as soon as your hand gets involved and can tell which glove is meant for your right hand the environment becomes chiral. So, the table = an achiral environment, and your right hand = a chiral environment.

Nature is a perfect example of chirality and all living systems are chiral environments. Examples of chiral molecules in our bodies include our amino acids and sugars on a small scale, which has larger impacts on organisms as a whole. If you were to take a look at every single honeysuckle plant you would find that they all climb by spiraling to the left, with no exceptions. Similarly all humans have their stomach on the left and liver on the right, and every double helix of DNA spirals clockwise. Nature has somehow selected a single enantiomeric form of each chiral molecule present in life, which remains consistent in all types of life with the exception of a few species of bacteria. How this has happened is still an ongoing source of debate within the scientific community as it is impossible to synthesize a single enantiomer if the starting material is not itself enantiomerically pure, and where there is no life there is not enantiomeric purity. 

An interesting side effect of chirality is the impact it has on plane polarised light. When many of the same enantiomers are together they interact with light, causing it to rotate. This is easiest to understand for molecules where there is only one chiral center, in which case one isomer will be dextrorotatory (rotate the plane clockwise), and the other isomer will be levorotatory (rotate the plane anticlockwise). The rotation will be of the same magnitude but in different directions for each enantiomer, thus a racemic mixture (50:50 ratio of enantiomers) is not optically active as the equal amounts of both enantiomers cancel out the plane rotation. 

It is not only light that responds differently to the enantiomers of a molecule, so do our bodies. Most of the time this is harmless, like how the molecule limonene can be detected by your scent receptors as smelling of sharp lemon or more rounded and orangey depending on the enantiomer present; similarly, spearmint and caraway seeds smell completely different despite being enantiomeric forms of carvone (a ketone). However, sometimes the presence of the incorrect enantiomer can have dramatic consequences, particularly when it comes to drugs. Parkinson’s disease can be treated using the amino acid dopa(3-(3,4-dihydroxyphenyl)aline, only the left handed enantiomer of this molecule is effective in restoring nerve function, the right handed enantiomer is not only ineffective but also highly toxic. Therefore drug companies have to come up with ways to separate isomers after synthesis if the starting chemicals themselves are not enantiomerically pure. An example of what happens when the impact of chiral molecules is not properly researched can be seen in the thalidomide tragedy of the late 1950’s. Thalidomide (C13H10N2O4) was originally a hypnotic drug that was then marketed to pregnant women to ease nausea and anxiety. The R-thalidomide was in fact an effective sedative, but the other S-thalidomide caused life altering deformities in the children who often suffered from a combination of undeveloped or missing limbs, deformities to the skeletal and organ systems and sensory impairments to name but a few. 

Evidently chirality has a lot to answer for in nature, and hopefully by understanding the behaviour of these molecules scientists can come closer to understanding the origin of life, as well as avoiding repeats of tragedies such as that seen in 1958.