[Protein-crystallography] How do CNS deal with the His NE2 atoms coordinated with Zn2+?

[Wei Zhao]

Dear all:
      we have a crystal structure with a Zn2+ tetrahedrally coordinated by 
three His NE2 atoms and one water. Is the proper coordinating distance between
three His NE2 atoms and Zn2+ ion is in the range from 1.95-2.1? But we have 
not get this result by CNS refinement with parameter_infile 'protein_rep.param and
ion.param'. We consider the His NE2 atoms are non-protonated when coordinated with
Zn2+, differ from the free His residues. But we don't know 
how CNS deal with the His NE2 atoms coordinated with Zn2+. 
How can we get the right coordinating distance?
 
Thank you very much for responds!
Any help is welcome!
 
Best regards,
 
Wei