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.
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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.
Perspectives
Dr Alfons Haedener, a specialist in organic biological chemistry, came on a research sabbatical to work with me in Manchester. We had worked extensively together over the years on the crystal structure and chemical catalysis properties of the active form of the enzyme hydroxymethylbilane synthase using time-resolved Laue crystallography. This past work (published in Faraday Transactions in 1998, in Acta Cryt D in 1999 and Faraday Discussions in 2003) was unrelated to the present study on crustacyanin. In taking an interest in our carotenoids research in biological and chemical crystallography (Prof John Helliwell, Dr Michele Cianci and Dr Madeleine Helliwell), Alfons became convinced of enolate formation as being very plausible, as did my organic chemistry colleagues in Manchester (Dr Ian Watt and Dr Andrew Regan with his project student Shamima Begum). We then set out to design experiments to test the hypothesis. The context was that there was a considerable expansion of the known carotenoid crystal structures from the work in our Lab and this ensemble of crystal structures provided an excellent platform to see directly a variety of crystal packing environments and conformations of the free carotenoids, and which did not induce the necessary color shifts. Dr Bo Durbeej in Linkoping had already taken up the computational theme and published several papers on crustacyanin and the color change. It was quite obvious to invite him together with his student, Olle Falklof, to join in with these new studies. Dr Michele Cianci, who had been my PhD student in Manchester was by now at the European Molecular Biology Laboratory in Hamburg and it was quite natural to involve him again in what was after all his PhD thesis project (and which had led to the 2002 PNAS paper cited above).
Professor John Richard Helliwell
University of Manchester
Read the Original
This page is a summary of: On the origin and variation of colors in lobster carapace, Physical Chemistry Chemical Physics, January 2015, Royal Society of Chemistry,
DOI: 10.1039/c4cp06124a.
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