X-Ray Crystallography

Recently hired as a Ramsey GRA Eminent Scholar, Dr. Wang is the director of a macromolecular crystallography center. This technique yields the complete atomic structure of metallobiomolecules that can be prepared as single crystals. Again, this satellite facility is available to MRIL collaborators by advance arrangement [Bi-Cheng Wang, UGa].

Acronyms, synonyms

• X-ray Diffraction

Measured physical quantities

• Scattering of x-rays by electrons

Information available

• Ultimately produce experimental electron density maps that may be interpreted to give three-dimensional atomic coordinates to an accuracy of ~0.1x the limiting resolution for non-hydrogen atoms
• Some information can be obtained about atomic identities

Information NOT available, limitations

• Unambiguous identification of atom types is difficult
• Oxidation states of metals, etc., cannot be determined
• Crystallographic analyses are not at atomic resolution
• Solution of phase problem can present serious obstacles to the structure determination

Examples of questions that can be answered

• Determination of the three-dimensional structure of a macromolecule or cofactor
• Identification of ligand binding sites and structural consequences of mutations

Major advantages

• In principle, the structure can be determined in the absence of any additional information (such as sequence, cofactor structure, etc.)

Major disadvantages

• Requires well-ordered crystals
• Need to solve the phase problem, which requires derivatives or anomalous scatterers
• Structural analyses are usually not at atomic resolution - this gives problems with atomic identities and geometry definition

Sample constraints

• Need crystals that are typically more than 0.1 mm on a side, which contain more than a few mg protein/crystal
• Need sufficient material (~10 mg or more) to screen enough conditions to find crystallization conditions
• For new structures, must be able to solve the phase problem experimentally