Pr John R. Helliwell (Structural Chemistry, University of Manchester et CCLRC Daresbury Laboratory) invité par Andy Thompson
Lundi 6 février à 11h - Salle du RdC du Pavillon d'Accueil
The coloration of the lobster shell, famously known from its colour change on cooking, derives from a complicated mix of astaxanthin carotenoid molecules and several proteins in complex. Crystals of various components have been known for many years but the breakthrough in structure determination came from use of softer X-rays of wavelength 2 Å targeting the increase in the sulphur anomalous scattering and the xenon LI absorption edge. One structure of the gene group of proteins was thus solved (Cianci et al 2001 Acta Cryst D) and this could be used for molecular replacement solution of the beta-crustacyanin dimer complex (Cianci et al 2002 PNAS USA). The alpha-crustacyanin complex of eight b-crustacyanins still eludes us although progress has been made (Chayen et al 2003 Acta Cryst D). Nevertheless the molecular tuning parameters causing the 100nm bathochromic shift of the b-crustacyanin are now known from our work and have stimulated considerable further research in theoretical and carotenoid chemistry. There are wider biological implications too, including the colours of rare lobsters, where site-specific amino acid changes could be a cause and much molecular biology for colour tuning would be possible. A colour based heat sensor could also be designed. Public interest has been especially strong not least because ’the’ question: ‘Why does a lobster change colour on cooking? is known to nearly everyone. Interactions with newspapers, radio and TV people, and finally science writers and children assigned this as a homework task, have been illuminating, rewarding and at times downright humorous!
There are methodological implications of the work having shown the viability of structure determination with softer X-rays. We will soon be entering a new era of ultra-bright X-ray Free Electron Lasers (XFELs) and where the ease of producing hard (1.0 Å X-rays may yet prove difficult or certainly at only a few ‘global’ centres. Single molecules are aimed to become ‘the sample’ at such intensities; I have proposed that we should allow for two-wavelength delta f ‘ measurements to locate specifically bound ‘marker atoms’(J R Helliwell J. Synch. Rad. 2004, 11, 1-3). There is also the ‘molecules blowing up’ problem. On the other hand single molecule structure determination may be easier than I indicate. More immediately the application of delta f ‘ MAD to protein powder diffraction for de novo structure determination would extend the crystal sample size frontier with existing 3rd generation SR sources, current and under construction. Thus crystallites in a powder form can be smaller than the single microcrystal protein crystallography. ‘Powder dispersive differences (PDD)’ ideas and a test case are described in Helliwell, Helliwell and Jones Acta Cryst A November 2005 including softer X-rays usage to increase the sample scattering efficiency and to spread out the powder lines separation (if linewidths can be preserved).
Prof. Helliwell will be happy to answer questions on his talk and also on other developments and applications described in any of his publications:
>Synchrotron radiation protein crystallography facilities, methods and applications (1976 to date);
>Neutron protein crystallography facilities, methods and applications (1995 to date);
>Protein crystal perfection and use of microgravity platforms (1987 to 1999).
His most recent references include publications in:
Chem. Soc Reviews (2004);
PNAS USA (2004);
Reports on Progress in Physics (2005);
Also his 1992 book ‘Macromolecular Crystallography with Synchrotron Radiation’ is now available in paperback (January 2005);
Various publications in Acta Cryst, J. Appl Cryst, J. Synchrotron Radiation
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