Bright red lobsters have their colour pigment freed when they are boiled.© B.mason / Alamy

"And, like a lobster boil'd, the morn/From black to red began to turn", wrote Samuel Butler in his seventeenth-century satire Hudibras. But no one knew exactly why the lobster changed colour when it was cooked ... until now.

The answer has come from a team of Dutch researchers who say that this striking culinary transformation has a subtle explanation that is rooted in quantum theory.

Chemists know that a lobster's colour is due to a pigment molecule called astaxanthin, which is attached to a protein called crustacyanin. Astaxanthin is one of the carotenoid pigments responsible for the bright red colours of many animals and plants, including those of oranges, tomatoes and some birds' feathers.

When a lobster is boiled, its crustacyanin proteins unwind in the heat and the astaxanthin pigment falls off. This 'free' astaxanthin is red, just like most other carotenoids, and gives the lobster its freshly-cooked colour. But chemists were mystified as to why live lobsters are blue-black.

Francesco Buda of Leiden University in the Netherlands and his colleagues, have recently worked out what happens to the astaxanthin when it is embedded in the crustacyanin molecule to change its colour. They report their research in the Journal of the American Chemical Society¹.

Red shift

Red pigments absorb blue and green light and reflect the red part of the spectrum. When astaxanthin is incorporated in crustacyanin, its absorption is shifted across the spectrum towards longer (redder) wavelengths. This means it absorbs all visible light, so the pigment appears black.

Something similar accounts for the different sensitivities to colour of the light-absorbing cone cells in our eyes. In these cells, a single pigment molecule (retinal) is 'tuned' to different wavelengths by a coat of protein.

One idea, discussed for many years, is that the molecular scaffolding of the crustacyanin molecule distorts the shape of the astaxanthin, allowing it to absorb light across the spectrum. This would be rather like altering the resonant frequency of a piano wire by making it shorter or tighter.

But Buda and colleagues have calculated the pigment's quantum energy states, which show that merely changing its shape accounts for only about a third of the shift in absorption wavelength. This alone cannot make the pigment turn from red to black.

Another proposal is that the pigment becomes electrically charged inside the protein. But the Dutch team's experiments, using a technique called nuclear magnetic resonance spectroscopy, rule this out.

Dark interference

Instead, the team followed up on the discovery in 2002 that astaxanthin molecules in the crustacyanin proteins are grouped in pairs that cross each other in an X-shape².

This pairing, the researchers' calculations show, means that the two molecules interfere with one another, like cross-talk between electrical signals in neighbouring wires, and this shifts their quantum energy states. That in turn alters the wavelength of light that they absorb, accounting for most of the blackness.

"It's surprising that it took such a long time to solve this problem," says Buda. But he admits it is only in the past five to ten years that computers have been able to handle the demanding quantum-mechanical calculations involved. 

• References

  1. Van Wijk A. A. C. et al. J. Am. Chem. Soc., 127. 1438 - 1445 (2005). | PubMed | ChemPort |
  2. Cianci M. et al. Proc. Natl Acad. Sci. USA, 99. 9795 - 9811 (2002). | Article | PubMed | ChemPort |