Some chemistry behind a tattoo
Tuesday 22 August 2023
Chemistry can sometimes appear in surprising places. Recently I was on a train from Sitges to Barcelona in Spain when I noticed that the person sitting opposite had a chemical structure tattooed on his left wrist.
I recognised the indole ring but did not actually recognise the exact compound. The indole ring numbering system starts at the nitrogen atom in the two-membered ring so the molecule contains an ethylamine group on the third carbon atom and a hydroxyl group on the fifth carbon atom.
A quick Google search shows that the common name is serotonin.
structural formula of serotonin
Perhaps more surprisingly, the search also revealed that serotonin tattoos are not that uncommon. Serotonin is a neurotransmitter and is thought to play an important role in regulating mood. Many drugs used to treat depression, such as fluoxetine (Prozac) and citalopram (Cipramil), are SSRI's (selective serotonin reuptake inhibitors) which increase serotonin levels in the brain. It may be that having a serotonin tattoo acts as a reminder to stay happy and calm.
As practice for making connections between different parts of the syllabus, you could use the structure of serotonin to ask your students the following questions.1. What is the molecular formula of serotonin?
C10H12N2O
2. What is the index of hydrogen deficiency (IHD) of serotonin?
Either
use the formula:
IHD of CxHy = (2x + 2 − y) / 2
remembering that O does not affect the IHD and for each N you need to add 1 to the number of C atoms and 1 to the number of H atoms so the compound effectively becomes C12H14 giving an IHD = 6
or
better and more simply, deduce it directly from the structure: There are four double bonds and there are two ring systems, all the rest of the molecule is saturated, which gives an IHD of 6.
(Note that determining the IHD of compounds is in the 2014 syllabus but it is not in the new 2023 syllabus. It has been replaced by degree of unsaturation but you only need to apply degree of unsaturation to hydrocarbons.)
3. Suggest why two of the carbon atoms in the indole ring are not numbered.
The two carbon atoms where the two rings join cannot undergo addition or substitution so no substituents can bond to them without destroying the indole ring system.
4. What class of amine is present in the ethylamine group?
It is a primary amine, −NRH2
5. Suggest why serotonin has both acid and base properties.
Acid: The non-bonding pair of electrons on the oxygen atom in the phenolic −OH group can delocalise with the electrons in the aromatic ring making the O−C bond stronger and the O−H bond weaker, so the phenolic group can donate a proton.
Base: The non-bonding pair of electrons on the N atom of the amine group can accept a proton.
6. 'Magic mushrooms' contain both psilocybin and psilocin.
Suggest why psilocybin is undergoing clinical trials to treat terminally ill patients who are experiencing depression or anxiety.
The ony differences between psilocybin and serotonin are that psilocybin contains a tertiary amine instead of a primary amine and a dihydrogen phosphate group instead of a hydroxyl group. Since it is a psychedelic drug and has a very similar structure to serotonin it suggests that it may act in a similar way to serotonin and affect mood by relieving depression and anxiety.
The trials are taking place at St Vincent's Hospital, Melbourne in Australia. You can read about this and see a video produced by the hospital here.