On Tuesday, July 24, 2012 10:22:18 am Nat Echols wrote: > On Tue, Jul 24, 2012 at 10:14 AM, Haytham Wahba <haytham_wa...@yahoo.com> > wrote: > > 1- if i have anomalous peak of unknown heavy atom, How can i identify this > > heavy atom in general. (different methods) > > > > 2- in my case, i see anomalous peak in heavy atom binding site (without any > > soaking). preliminary i did mass spec. i got Zn++ and Cu, How can i know > > which one give the anomalous peak in my protein. > > > > 3- there is way to know if i have Cu+ or Cu++. > > You may be able to identify the element based on the coordination > geometry - I'm assuming (perhaps incorrectly) that it is actually > different for Cu and Zn. Marjorie Harding has written extensively on > the geometry of ion binding: > > http://tanna.bch.ed.ac.uk/ > > The only way to be certain crystallographically, if you have easy > access to a synchrotron, is to collect data above and below the K edge > of any candidate element, and compare the difference maps. (For > monovalent ions it is more complicated, since they don't have > accessible K edges.) On a home source, Cu should have a larger > anomalous map peak, but I'm not sure if this will be enough to > identify it conclusively.
As to the SR experiment - yes. As to the home source - no. Neither Cu nor Zn has appreciable anomalous signal when excited with a Cu K-alpha home source. http://www.bmsc.washington.edu/scatter An element's emission edge (Cu K-alpha in this case) is about 1 keV below the corresponding absorption edge. This makes sense, because after absorbing a photon it can only emit at an equal or lower energy, not a higher energy. So you can't reach the Cu absorption edge, where the anomalous signal is, by exciting with Cu K-alpha. Ethan -- Ethan A Merritt Biomolecular Structure Center, K-428 Health Sciences Bldg University of Washington, Seattle 98195-7742
