On the Origin and Variation of Colors in Lobster Carapace
What is it about?
Astaxanthin is a high melting waxy naturally occurring organic pigment, a pink-orange colour in its native form and giving to similarly coloured solutions. It is present as a protein complex in the shells of lobsters, and there it is responsible for the deep blue-purple of the live lobster. An X-ray crystal structure showing the binding of the astaxanthin was published in 2002 by us but not of sufficient resolution ie detail to show the exact charge state form of the bound astaxanthin (M. Cianci, P.J. Rizkallah, A. Olczak, J. Raftery, N.E. Chayen, P.F. Zagalsky and J.R. Helliwell “The molecular basis of the coloration mechanism in lobster shell: β-crustacyanin at 3.2 Å resolution” (2002) PNAS USA 99, 9795-9800.). As our introduction to our PCCP article nicely explains the context of our study we repeat it here:- ”Quantum chemical calculations and experiment suggested that neither conformational change nor exciton coupling can account for more than 30% of the observed colour shift, with the larger contribution arising from co-planarization of the end rings (ie which extends the number of alternating single and double bonds in the carotenoid from 9 to 13, a well known way that carotenoids of different lengths show different colours). Refs 8–10 Kuhn and Sorensen suggested in 1938 a second theme, Ref 11 which remained largely neglected, involving a reversible ionization of the astaxanthins upon complexation.” We now present evidence that the astaxanthin is present in these complexes in the form of a negatively charged ion (enolate) and that this anionic form is the majority origin of the large bathochromic shift in its visible light absorption maximum to give the dark blue colour of the live lobster.
Why is it important?
Over the last thirteen years since our X-ray crystal structure there have been competing groups studying this coloration mechanism, but hopefully now the issue is solved. It is a scientific curiosity, but it may also have important applications in the real world. The coloration is quite a complex process to do with the 3 dimensional structure of the proteins in complex with the astaxanthins it binds, and the implications could be very useful. For example astaxanthin is an antioxidant, so it has many health properties. But because it is insoluble in water the problem is how to deliver it to a target. But our findings suggest that mixing it with crustacyanin could do that and allow the astaxanthin to get to a target such as via the stomach. It could also be used as food dye, for example to help create blue coloured ice cream. Or it could be used in food stuffs to help people know when food has been cooked properly; a dot on the food that changes colour when it reaches a certain temperature could be used. Most fundamental of all is arousing the curiosity of children and the public in basic science and our marine environment. In the era of climate change it is important for all to think about the delicate nature of life and the sustainability of life on the planet. How and why has lobster evolved this elaborate and delicate coloration mechanism? It is a beautiful and yet intriguing phenomenon.
The following have contributed to this page: Professor John Richard Helliwell
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