Re: [ccp4bb]: unknown cation at 2.6-3.0A bonding distances

["David J. Schuller" <djs63@cornell.edu>]

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On Fri, 4 Oct 2002, Aleks Roszak wrote:

> Could you please suggest any good site, database or so, in which I could find
> details of coordination (bond distances, coordination geometry, etc) for some
> light metal ions, or even better any single atom ions.
> 
> I seem to have an ion [quite spherical density up to 6sigma, definitely much
> higher than water, refines to B=2.0 while average B for this structure is 26
> (from TLS refinement)] coordinated by 3 main-chain carbonyl oxygens of a loop,
> and possibly also by 2 water molecules, at distances of 2.6-3.0A. The three
> carbonyl oxygens plus one water molecule oxygen form approx. a (relatively
> flat)
> pyramid with the unknown ion at the apex.
> 
> Any suggestions are appreciated, thanks.

The very low B indicates that the atomic number is probably higher than 
what you have in there now.

IS there any biochemical evidence? Atomic Absortion Spectroscopy, for 
instance?

Did you add any metals as part of  the crystallization conditions?

I think Zn tends to be tetrahedral, so maybe Ca might be a good first 
guess.

Anomalous scattering - most metals scatter anomalously, and the 
wavelengths are characteristic.  For example, Zn has a peak at 1.28 A, so 
if it doesn't show up in Cu rotating anode data (1.54 A) but does at 
typical synchrotron wavelengths (~ 1.0 A) Zn is a good bet.  Ca would show 
up at 1.54 and above.

I would recommend the U. Washington Anomalous Scattering periodic table;
http://www.bmsc.washington.edu/scatter/AS_index.html

cheers,

=======================================================================
"Being a true scientist, Ralph wanted to make his own dangerous experiments."
                                                      - Hugo Gernsback
=======================================================================
                        David J. Schuller
                        modern man in a post-modern world
                        MacCHESS, Cornell University
                        djs63@cornell.edu