Veerendra, You can rule out if zinc has replaced calcium ions (although I agree with Nat and others that looking at the coordination sphere should give a big clue) by taking a few crystals, washing them a couple of times and subjecting them to ICP-MS analysis, if you have access to this technique. You can learn how many zinc, if any, have bound per one protein molecule in the dissolved crystal.
Best Chitta ----- Original Message ----- From: "Veerendra Kumar" <veerendra.ku...@uconn.edu> To: CCP4BB@JISCMAIL.AC.UK Sent: Tuesday, October 30, 2012 2:55:33 PM 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
