Hi David, You can use Sheldrick's Calcium Bond Valence Sum to descriminate between metals (see Muller, P., Kopke, S., and Sheldrick, G. M. (2003) Acta Crystallogr., Sect. D: Biol. Crystallogr. 59, 32-37) even at low resolution. I have had good success with this method combined with estimation of anomalous contribution scaled against the S atoms as suggested by Kay.
Have a look at <gratuitous self plug> Graham, S. C., Bond, C. S., Freeman, H. C., and Guss, J. M. (2005) Biochemistry 44, 13820-13836. </gratuitous self plug> for plenty of examples. To do the anomalous difference calculations I just integrated a 2x2x2 A box around the metal atom and around the S atoms (in MAPMAN) then calculated the ratio of anomalous difference... Cheers, Stephen On 4/16/07, David Briggs <[EMAIL PROTECTED]> wrote:
Dear all. I have recently solved a structure in-house, 2.8A, CuKa. I have a metal ion bound very obvious hepta-valent co-ordination, which would suggest either Ca or Mn. Neither was present in the crystallisation setup, but there was some Mg around, which has contaminants of both Ca & Mn. At 2.8A, I don't really think I can reliably discriminate between 2.15A & 2.36A distances to coordinating atoms (http://tanna.bch.ed.ac.uk/newtargs_06.html ). The B factors for refined Ca are 18, and Mn 30. The B-factors of coordinating atoms vary from... 18 > 30 - so no help there. I have a nice clear 6sigma anomalous difference peak, but then, according to http://skuld.bmsc.washington.edu/scatter/ both Ca (f" ~1.3) and Mn (f" ~2.8) scatter anomalously at that wavelength. The obvious solution is go to a synchrotron and scan around the Mn edge and see what happens, however, whilst waiting for beam time, is there any way I could... oh I don't know, use the peak in my anomalous difference Fourier to figure out what anomalous signal would be required to generate a peak of that size - a sort of back-transform??? Is this do-able, and if so, how would one go about it? Cheers, Dave -- --------------------------------------- David Briggs, PhD. Father & Crystallographer www.dbriggs.talktalk.net iChat AIM ID: DBassophile --------------------------------------- Anyone who is capable of getting themselves made President should on no account be allowed to do the job. - Douglas Adams
-- Dr Stephen Graham Nuffield Medical Fellow Division of Structural Biology Wellcome Trust Centre for Human Genetics Roosevelt Drive Oxford OX3 7BN United Kingdom Phone: +44 1865 287 549
