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