It is my feeling that the surface binding sites for ions are not so rigid as to be unable to accommodate the larger ionic radii of heavy metal compounds. In part this is why halide soaks, particularly iodide salts, are so successful as heavy atom derivatives (Dauter, et al., 2000, Nagem et al., 2003; Dauter & Dauter, 2006)). On the other had, it is my experience that while many anion binding sites can be observed, via anomalous difference maps, very few cations are observed. See:
Mark Andrew White, Natalia Mast, Ingemar Bjorkhem, Eric F. Johnson, C. David Stout, and Irina A. Pikuleva, The Use of Complementary Cation and Anion heavy-atom salt derivatives to solve the structure of cytochrome P450 46A1, Acta Cryst. D 2008; 65(16);487-95 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2467524/pdf/d-64-00487.pdf The problem is that Absence of proof is not proof of absence. The crystallographic water molecule has traditionally been marker for any solvent entity that is not readily identifiable and round in shape. Most crystallographers understand this, anh I am sure that most biologist do not. Sorry to muddy the waters, or is that ions? Mark On Thu, 2010-02-18 at 08:58 -0600, Thomas Womack wrote: > The deposition 3fiy from the start of last year might be of interest: > > FORMUL 2 NA 199(NA 1+) > > FORMUL 20 HOH *256(H2 O) > > It is annoying that the periodic table offers such a discrete range of sizes > for 1+ ions; I hoped the lanthanide contraction would provide a heavy sodium > substitute with lots of anomalous scattering, but (if I believe > http://en.wikipedia.org/wiki/Ionic_radius) no ... Ag+ is the closest match in > size (still 15% or so bigger) but silver(I) compounds are usually insoluble, > La3+ is the same size as Na+, and LaCl3 nicely soluble, but obviously it > coordinates very differently. > > Tom Yours sincerely, Mark A. White, Ph.D. Assistant Professor of Biochemistry and Molecular Biology, Manager, Sealy Center for Structural Biology and Molecular Biophysics X-ray Crystallography Laboratory, Basic Science Building, Room 6.660 C University of Texas Medical Branch Galveston, TX 77555-0647 Tel. (409) 747-4747 Cell. (409) 539-9138 Fax. (409) 747-4745 mailto://wh...@xray.utmb.edu http://xray.utmb.edu http://xray.utmb.edu/~white
