Do you notice anything about the molecules of lactic acid pictured? It’s not an optical illusion, they are mirror images of one another and if you try to place one on top of one another, you will find that they do not overlap. It is just like how your left and right hands are the same but are opposite of one another. When this happens in molecular compounds in chemistry, they are called an enantiomer.
When each half of an enantiomer pair are placed in mirrored environments, they have the same chemical and physical properties except that they rotate polarised light in opposite directions. But life isn’t a series of mirrors so in a real situation, enantiomer pairs react differently with the same substance. In the world of medicine, the working drug is often an enantiomer where one will treat the symptoms well and the other one can cause unwanted side effects, some of them being quite severe.
From 1957 to 1061, a sedative, thalidomide was sold across the world as a wonder drug. It solved the problem of insomnia, coughs and colds. It also suppressed the symptoms of morning sickness and thousands of women took the drug. At the time it was not thought that the medications that an expectant mother took would pass through the placenta to the developing baby. It did not take long to link an increase in birth defects to thalidomide and soon it the wonder drug was labelled as “one of the biggest medical tragedies of modern times“. Only estimates have been made of the total number of victims affected by thalidomide. It is thought that around 10,000 people were affected.
Since this event, medications and pesticides are tested under stricter requirements before being approved for use. In recent times, thalidomide is the subject of medical research for the treatment of myeloma.
Update and a REALLY Important Note:
Soon after I posted this blog post, I was alerted by Stuart Cantrill to a Chemistry urban legend that I had unwittingly helped to spread. Did you spot it? No? Ask yourself this question. If the offending enantiomer of thalidomide had been identified, why didn’t the medical authorities just give people the one that helped people without the side effects? After all it would be great if people suffered less from insomnia and pregnant women suffering from morning sickness would be most grateful.
The risk of the tragedy repeating itself would be too high. At least that’s what my Chemistry teacher told me in high school. And then my Chemistry lecturers said the same thing at university. It didn’t sit well with me but with so many Chemists saying the same thing, I soon believed it without questioning it again. That is until recently.
There is an answer to why the correct form of thalidomide, ((R)-thalidomide), is not a routine medication for insomnia or morning sickness in 2011. It it is related to the risk of a tragedy repeating. The tragedy has an absolute certainty of repeating. This is because the enantiomer pairs of thalidomide can transform into its mirror image when within the human body. So even if only (R)-thalidomide was given to someone as medication, once inside the human body, it would start to form (S)-thalidomide, the one responsible for birth defects.
When an enantiomer does this whether it be in a living organism or in a beaker, it is called racemisation.
You can read more about the thalidomide chemistry urban myth as well as others here, (it is behind a paywall).