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Hi... MicroPIXE (Particle Induced X-ray Emission) will sort you out... Check out these papers from Elspeth Garman: Structure. 1999 Dec 15;7(12):R291-9 Prog Biophys Mol Biol. 2005 Oct;89(2):173-205. (Just PUBMED for MicroPIXE) This should give you a definitive answer... HTH Dave On 10/5/06 12:30 am, "Wendy Gordon" <[EMAIL PROTECTED]> thought: > *** For details on how to be removed from this list visit the *** > *** CCP4 home page http://www.ccp4.ac.uk *** > > > Hello- > Thanks for entertaining my non-ccp4 question... > > I am refining a crystal structure at ~2 Angstroms resolution in which I > find a large, unexpected electron density that is potentially a Zinc > ion because it is coordinated by 2 Histidines, 1 Glutamic acid, and > possibly a water molecule. > > This site appears as a strong peak in anomalous difference Patterson > maps (the data was collected at SelMet wavelength 0.979 Angstroms and I > believe that Zn's absorption edge occurs around 1.2 Angstroms). The > problem is, I didn't add any zinc in any purification or > crystallization conditions. I DID affinity purify the protein with > nickel beads, so potentially it could be a Nickel ion. I should also > say that during refinement in refmac where my Zn occupancy is held at > 1, that I obtain a negative peak in the 2Fo-Fc in this position, but if > I leave the site unoccupied- I get a huge positive peak- so I either > have the wrong species defined or my occupancy is not 100%- right? > > Is there any way short of biochemical means (ITC, mutation, etc.) to > figure out what species is occupying this electron density? I have > thought of atomic absorption- has anyone tried it to determine the > metal species in a protein? Does it seem possible that I could have a > Zn ion in my protein crystal where the Zn could only come from our > standard DI water supply? > > Thanks so much! > Wendy Ryan Gordon -- *************************************************************** Dr David Briggs | Structural Biology Lab (309) | Tel : (+44)(0)20 7269 3360 Cancer Research UK | 44 Lincoln's Inn Fields | Holborn | London | UK | WC2A 3PX | ****************************************************************
