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As Ethan mentions, your best bet for an 100% positive ID would be to collect a dataset above and below the Mn edge and see the anomalous signal from the Mn atom disappear. If that is not an option, there is a great paper by George Sheldrick and friends regards using the Calcium Bond Valence Sum to determine the nature of a metal based on its coordination geometry. [P. Müller, S. Köpke and G. M. Sheldrick (2003) Is the bond-valence method able to identify metal atoms in protein structures? Acta Cryst. D59, 32-37]. <shameless self-plug> You could also have a look at one of my papers where we applied this (and integration of the anomalous difference density around the position of the scatterer, scaled against the Sulfur signal of the Met's and Cys'es in the protein) to confirm metal loading of a metalloenzyme [Graham, S, Bond, C, Freeman, H, Guss, J. (2005) Structural and functional implications of metal ion selection in aminopeptidase P, a metalloprotease with a dinuclear metal center. Biochemistry.44: 13820-36] Good luck, Stephen On 2/29/08, Sun Tang <[log in to unmask]> wrote: > Dear All, > > In my structures, I want to assign Mn or Ca ions for some densities. But > when I did not have anomalous density in CCP4i. I am not sure whether I was > correct. The following was what I did: > > I processed the data with HKL2000 and select anomalous signal in scaling. In > CCP4i, I selected "Run FFT-Creat Map" in the "Map& Mask Utilities". I select > "O format to cover asymmetric unit" and "Plot section on Z axis from 0 to 1 > in steps on 10". All others were by default values. I display in ono10. > > I collected the data at the wavelength of 1 A. Do I need to adjust the > wavelength to maximize the anomalous signal from Mn or Ca? > > Any ideas and suggestions are greatly appreciated! > > Sun Tang > > ________________________________ > Never miss a thing. Make Yahoo your homepage. > > -- 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