Heavy Metal Soaking

Heavy metal soaking

Several techniques are used to solve macromolecule crystal structures. To overcome phase problems, one possibility is to soak the protein crystal in a heavy atom solution

Safety & Precautions

Heavy atoms are toxic to humans, and uranyl compounds are radioactive.

When preparing heavy atom derivatives, special care must be taken to prevent poisoning/irradiating yourself or others

Wear gloves and avoid direct contact with the skin.
If contact occurs, wash the contaminated area thoroughly with soap and water.
Promptly clean up spills.

Please take the time to read the complete heavy metal handling, safety, and disposal protocol

Choose the right heavy atom

Cystein, hisitidine, and methionine are reactive with Class B heavy atoms (Hg, Au, Pt, Ir, etc.)
Glutamic acid and Aspartic acid are reactive toward Class A heavy atoms (Lanthanides, Actinides, e.g. U, Sm)
Lanthanides and Actinides tend to have the strongest anomalous signal.

To check how many anomalous electrons are in a given element see Ethan Merritt's X-ray Anomalous Scattering Tables

Consider how heavy atom reactivity varies with pH

Useful links

Petsko, G.A., "Perparation of Isomorphous Heavy-Atom Derivatives Methods in Enzymology, Volume 114, pages 147-157
Blundel & Johnson's book, "Protein Crystallography" pages 183-239
Recipes for iodide and cesium derivatives
Phasing with potassium iodide- a powerpoint presentation
Recipe for selenomethionyl derivatives

Table of common heavy-atom derivatizing reagents

Screening for heavy-atom derivatives and obtaining accurate isomorphous differences

M.A. Rould, Methods in Enzymology, 276, pp 461-72 (1997)