Tim
I did a survey of the literature some time ago and used Harding, Acta
Cryst. D62 (2006), 678-682 for the data used in
phenix.metal_coordination. I recently added the data from Zheng et al.
Journal of Inorganic Biochemistry 102 (2008) 1765-1776 and plan to
include it soon.
You are quite correct that the coordination number affects the bond
length. This is currently ignored in PHENIX but it may well be useful.
I'll read Shannon and Prewitt with a view to improving the
coordination code.
One thing you can do is add the slack option to the edits giving the
bond potential a flat bottom. This allow complete freedom for the
metal to use the electron density for fitting within the slack
distance of the ideal.
refinement.geometry_restraints.edits {
bond {
action = *add
atom_selection_1 = chain F and resid 1 and name FE1
atom_selection_2 = chain A and resid 69 and name SG
distance_ideal = 2.35
sigma = 0.1
slack = 0.1
}
}
I'm very interested in discussing your situation and making it better.
If you send me your model (off line) we can investigate a better
solution.
Nigel
NB. Any files sent to me will be held in strictest confidence.
On Tue, Nov 22, 2011 at 2:48 PM, Timothy Springer
Hi, We are refining structures with Ca binding sites that are replaced by Mn. Using the coordination restraints in Phenix (distance only) obtained with phenix.metal_coordination, we wind up with the ligands coming somewhat too close, so there is positive density on the opposite side of the ligating residue from the metal. We may have a series of problems, including incomplete replacement of Ca by Mn. However, the bigger issue I want to address is coordination number. In the work by Shannon and Prewitt, such as Acta Cryst. B25, 925, metal oxides are examined, and the point is made that the Mn-O distance with a coordination number of 6 is shorter than for 7. Ca commonly has either 6 or 7 ligands, and with 7 ligands (most common) the O cannot squeeze as tightly together and are farther away from Ca. Mn more commonly has 6 ligands, but can have 7. What are the distance restraints in Phenix based on? They seem similar to those in Harding MM, Acta Cryst D62, 678 (2006). This paper lumps measurements for different coordination numbers together for Mn-O and Ca-O distances, but not for Co, Cu, and Zn. It does seem to help to add metal ion restraints in Phenix. Things don't seem to come out as well without them. Would it be helpful to treat 6 and 7-coordinate metals differently? Or should we just loosen the restraints? How does Refmac treat metals? One does not need to add restraints in its case. With my best regards, Tim Timothy A. Springer, Ph.D. Latham Family Professor of Biological Chemistry and Molecular Pharmacology
Harvard Medical School http://idi.harvard.edu/springer Immune Disease Institute [email protected] Program in Cellular and Molecular Medicine, Dept. Medicine Div. Hematology Children's Hospital Boston 3 Blackfan Circle, Rm 3103 phone: 617-713-8200 Boston MA 02115 fax: 617-713-8232
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