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This is from Ethan's very useful site, which should be known to everyone doing protein work at synchrotron sources:

http://skuld.bmsc.washington.edu/scatter/

http://skuld.bmsc.washington.edu/scatter/AS_form.html

The usual caveats apply... these calculations, especially for f", are often not especially accurate very near the peak. Chemistry causes small shifts in edge energies compared to the elemental state, and f" is often larger than the calculated value.  Still, you can see that your experiment was conducted close to and above (energy) the Fe K-edge,  Accordingly Fe is expected to make nontrivial dispersive and anomalous contributions at this energy.

On Aug 9, 2011, at 11:38 AM, Huiming Li wrote:

Hi All,

  I am working on a Tb binding protein on which I collected anomalous data at Tb edge of 1.648 A.  Each protein is designed to bind one Tb. There are two copies of the protein in an ASU. I have two questions. First, I am only able to see one copy of the protein with Tb bound, and no density on the other copy.  Isn't this a bit surprising? Second, there is one additional peak on each monomer at the site where Fe is known to bind, and Fe has an edge of 1.739A. At 1.648A, f' and f'' of Fe is only about 1/5 of Tb. Is it possible Fe also shows some anomalous signal at Tb edge?

Thank you,

Huiming Li, Ph.D.
Immune Disease Institute
Children's Hospital Boston
Harvard Medical School
Boston, MA 02115