It's possibly a transition metal ion. Zinc is a common adventitious
contaminant of solutions. Typical Zn-O distances (tetrahedral or
pseudo-tetrahedral coordination) are 2.0 A. ICP-OES or ICP-MS of the
protein solution might offer a clue to the possible identity of the
metal ion, since it appears to be nearly stoichiometric with your protein.
_______________________________________
Roger S. Rowlett
Gordon & Dorothy Kline Professor
Department of Chemistry
Colgate University
13 Oak Drive
Hamilton, NY 13346
tel: (315)-228-7245
ofc: (315)-228-7395
fax: (315)-228-7935
email: rrowl...@colgate.edu
On 2/4/2013 11:33 AM, Jared Pitts wrote:
Hello all,
I have recently solved a 2.0 angstrom resolution structure. The
structure is near complete but I have some unusual density at the
crystallographic interface between two chains of different asymmetric
units. The linked photos show the density at with a Fo-Fc at 3 sigma
and 2Fo-Fc at 1 sigma.
http://s1359.beta.photobucket.com/user/jatmp06/media/asp210b3_zpsfc64f0cb.png.html?sort=3&o=1
http://s1359.beta.photobucket.com/user/jatmp06/media/asp310b2_zps5a52816a.png.html?sort=3&o=0
The center of the density is approximately 2.1 angtroms from both the
Asp sidechain and the carbonyl oxygen on the other chain. I have
attempted to fit in either calcium or sodium atom and waters, but
these attempts don't satisfy the positive density while maintaining
proper coordination.
My buffer components are PIPES, NaCl, TCEP and the crystallization
conditions are PEG 1K, Calcium acetate, Imidazole pH 8.0, and
glycerol. Any insight as to what this density could be would be
greatly appreciated.
Thanks,
Jared Pitts
Jared Pitts
Doctoral Student
Department of Molecular Biology and Microbiology
Tufts University School of Medicine
136 Harrison Avenue
Arnold Building Room 611
Boston, MA 02139