On Tuesday, July 24, 2012 10:22:18 am Nat Echols wrote:
> On Tue, Jul 24, 2012 at 10:14 AM, Haytham Wahba <[log in to unmask]> 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
|