[ccp4bb]: Mn versus Mg..Continues

[raji <raji@lamar.colostate.edu>]

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Hi All,
Thanks to everyone who already replied my email.
All prior work done by my PI and all other structures from our lab have 
reported these ions as Mn.
Recent publications from another lab have shown unambiguously that these ions 
are indeed Mn+2. Here's the paper:
Proc Natl Acad Sci U S A  2002 Aug 20;99(17):11169-74 
DNA-dependent divalent cation binding in the nucleosome core particle.
Davey CA, Richmond TJ.
The authors identified the ions as Mn2+ ions by calculation of a difference 
Fourier map by using anomalous dispersion differences for structure factor 
amplitudes and phases corresponding to the refined atomic model lacking metal 
ions. In addition, these ions are bound with very high occupancies.
I am fairly convinced the DNA-bound ions are Mn+2.
Also, we DO NOT have Mg in our buffers.
I ended up refining them as Mg by MISTAKE one time instead of writing them as 
Mn and that's when I noticed that CNS seemed to handling it better as Mg.
Im still not clear about what's going on.
Why would putting in Mg help Rfac & Rfree, if the # electrons Mg = # electrons 
Water ?
Could someone pl. share your thoughts.
Thanks for your immediate responses.
Raji

Rajeswari Edayathumangalam
Dept. of Biochemistry & Molecular Biology
Colorado State University
Fort Collins, CO 80523 USA