March 05, 2021

Coffee is a popular brewed beverage all over the world and for some, it’s even a daily staple. C. arabica is the predominant variety of coffee bean and some years ago, I was fortunate to see coffee trees at a plantation in Costa Rica. After harvesting the beans, they dry them on the ground under the sun before roasting.

The main stimulant in coffee, the one that perks you up and makes you feel awake, is caffeine. The caffeine molecule is what chemists call a methylated xanthine alkaloid. A methyl group is that -CH₃ (carbon attached to 3 hydrogens) you commonly see on many different molecules. Caffeine has three methyl groups attached to three different N (nitrogen) atoms. See what it looks like on this Coffee Tote Bag.

Now look at the molecule on the Chocolate Tote Bag. You may have to do a double take. It looks very similar to caffeine but there's a difference. Can you spot it?

Number of Methyl Groups

Theobromine is the key chemical component in chocolate and is structurally very similar to caffeine. Also a methylated xanthine alkaloid, except that it has two methyl groups instead of three. So, that extra methyl group in caffeine is enough to give coffee its distinctive physiological impact of keeping us alert, that chocolate doesn’t do.

Position of Methyl Groups

What if you changed the position of the methyl groups on the same molecule? What would happen?

The diagram above shows the molecular structures of theobromine and theophylline, both of which are present in cocoa beans. Both molecules have the exact same chemical formula C₇H₈N₄O₂and the same number and types of atoms. However, the positions of the methyl groups are different.

By simply changing the position of the methyl groups on theobromine, it becomes the molecule theophylline, which has a completely different function than theobromine. Theophylline is a medicine used to treat respiratory illnesses such as COPD (chronic obstructive pulmonary disease) and asthma. It is a vasodilator, meaning that it widens our blood vessels.

Isn't it fascinating that small differences in a molecule, such as changing the number and position of methyl groups can drastically change their chemical properties, as well as their physiological impact in the human body?

That's the story of three methylated xanthine alkaloids: caffeine, theobromine and theophylline. Very similar yet so different.

What do you think? Are you curious about anything else? Let me know in the comments below.