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If you calculate an edgeplot via Ethan's server: http://skuld.bmsc.washington.edu/scatter/AS_form.html you'll see that Zn @1Å has about 3 anomalous electrons whereas Ca less than 1, so assuming occupancy of 1 the stronger anomalous signal should give you a hint, second looking at the refined B-values if you placed the wrong metal should also coincide with your correct choice of metal. The question with the mixture, well lets wave some hands and bring in biological relevance. If the protein is supposed to do something with Ca then I would say so, even if Zn is bound. If the anomalous signal is comparable to the SeMet sites then I would model Zn there but say it's likely not physiological as Ca is required by this enzyme. Jürgen On Oct 30, 2012, at 3:12 PM, Jim Pflugrath wrote: > How would you distinguish between a mixture of Ca and Zn in the same locations? > > ________________________________________ > From: CCP4 bulletin board [[log in to unmask]] on behalf of Kumar, Veerendra [[log in to unmask]] > Sent: Tuesday, October 30, 2012 1:55 PM > To: [log in to unmask] > Subject: [ccp4bb] Ca or Zn > > Dear CCP4bb users, > > I am working on a Ca2+ binding protein. it has 4-5 ca2+ binding sites. I purified the protein in presence of Ca2+ and crystallized the Ca2+ bound protein. I got crystal and solved the structure by SAD phasing at 2.1A resolution. I can see the clear density in the difference map for metals at the expected binding sites. However I had ZnCl2 in the crystallization conditions. Now i am not sure whether the observed density is for Ca or Zn or how many of them are ca or zn? Since Ca (20 elctron) and Zn (30 electron), is this value difference can be used to make a guess about different ions? > is there any way we can find the electron density value at different peaks? > > Thank you > > Veerendra ...................... Jürgen Bosch Johns Hopkins University Bloomberg School of Public Health Department of Biochemistry & Molecular Biology Johns Hopkins Malaria Research Institute 615 North Wolfe Street, W8708 Baltimore, MD 21205 Office: +1-410-614-4742 Lab: +1-410-614-4894 Fax: +1-410-955-2926 http://lupo.jhsph.edu