The Chemistry of Egg Fluorescence

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Here’s something to try over the Easter weekend: take a UV light and shine it on some eggs. You’ll be rewarded with a red glow from their shells. What causes this red glow? That’s what this graphic looks at!

The red glow, or fluorescence, of eggs under a UV light is due to a particular molecule found in egg shells. This molecule is protoporphyrin IX (PPIX). PPIX is a precursor of haemoglobin, the pigment that gives our blood (and that of chickens) its red colour. How, then, does this molecule get on to eggshells?
 
PPIX converts into haem, the non-protein component of haemoglobin, by the action of a particular enzyme. This enzyme’s activity is low in the oviduct of some birds, including chickens. As a consequence of this, PPIX is incorporated into egg shells, given them their brown colour.
 
PPIX causes egg shells to fluoresce under UV light. The fluorescence is due to electrons in PPIX molecules absorbing the UV light. This pushes the electrons to a higher energy ‘excited state’, which is short-lived. First, some of the excess energy is lost to the surroundings, for example as heat. Then, the electrons fall back to their original energy level, getting rid of their excess energy as visible light. This is seen as fluorescence. From start to finish, the whole process is over in billionths of a second.
 
There’s a more detailed explanation of the process of fluorescence here. The image below shows the fluorescence of the dimethyl ester of PPIX (used because it has better solubility and stability). The images below (and those in the main graphic) were kindly provided by Hannes Hiller, who shares beautiful chemistry images over at MicroXtals.
 
The Chemistry of Egg Fluorescence pt 2
 
Note: You should have no problem with using UV torches to reproduce the fluorescence seen in the pictures in this post. Note that if you’re trying to reproduce the effect in a laboratory, the normal UV-A light source used in labs is 365 nm – this wavelength is insufficient because PPIX has almost no absorption around this wavelength. So it’s important to use the “cheap” UV sources (395 to 405 nm) to reproduce the results!