Today I have a chemical conundrum for you: My cat was recently prescribed Clavamox (consisting of amoxicillin trihydrate and clavulanate potassium), and the vet warned me to keep the pills in their opaque packaging until their use because they turn brown and disgusting (and presumably become less effective) when exposed to light. I have of course heard of light-sensitive drugs in the form of photodynamic therapy, or in regards to the idea that milk should be protected from light, but I hadn’t run across light-sensitive antibiotics (not sure whether that’s a testament to my poor chemical knowledge or my good health). From a very speedy search, it seems that amoxicillin, and not clavulanate, is the delicate compound in the mix.
So, the question for you all, as I am too lazy (I mean busy. Busy!) to figure it out myself, is: why is amoxicillin light-sensitive?
Catherine (associate editor, Nature Chemical Biology)
Well, amoxycillin does have an aromatic ring, and delocalized systems like that tend to absorb light in the UV/Vis range. Maybe that has something to do with it.
Hi Cathy, here’s another possibility:
I guess the carbonyl group of the b-lactam in amoxicillin can undergo a Norrish type I reaction and thus release the strain of the 4-membered ring. The resulting radical aldehyde can then substract a hydrogen radical from the aza-thiophene ring and give rise to a hydro-aza thiophene and a free aldehyde group. Amoxicillin therefore loses the antibiotic b-lactam ring.
Thanks Miguel! A very satisfying answer. Now I will have to go review Norrish reactions…