Anomalous dispersion

Definition

Anomalous dispersion occurs when the refractive index of a material varies significantly with changes in wavelength, particularly in the context of X-ray diffraction. This phenomenon is particularly useful in crystallography as it enhances the contrast in scattering factors, allowing for more accurate determination of crystal structures by making certain atoms more distinguishable than others.

5 Must Know Facts For Your Next Test

1. Anomalous dispersion can significantly enhance the contrast between different types of atoms within a crystal, making it easier to determine their positions.
2. The effectiveness of anomalous dispersion is often utilized in methods like MAD to solve complex crystal structures, particularly for proteins.
3. It arises from resonant scattering effects near an atom's absorption edge, causing variations in the scattering factor with wavelength.
4. The use of anomalous dispersion is critical for locating heavy atoms or metal ions in macromolecular structures, aiding in experimental phase determination.
5. Understanding anomalous dispersion allows researchers to improve data collection strategies and refine models for accurate structural analysis.

Review Questions

• How does anomalous dispersion impact the determination of atomic positions in crystallography?
  • Anomalous dispersion enhances the visibility of certain atoms within a crystal by increasing the contrast in their scattering factors. This means that when X-rays are used to analyze a crystal, the atoms that exhibit significant anomalous scattering can be more easily distinguished from those that do not. As a result, researchers can accurately pinpoint atomic positions, which is crucial for building reliable structural models.
• Discuss how the phenomenon of anomalous dispersion contributes to solving the phase problem in protein crystallography.
  • Anomalous dispersion plays a vital role in addressing the phase problem by providing additional information about the phases of scattered X-rays. By exploiting differences in scattering due to anomalous dispersion, particularly from heavy atoms or specific elements, crystallographers can derive phase information that is typically lost. This information helps reconstruct electron density maps more accurately, facilitating better interpretation of protein structures.
• Evaluate the advantages and limitations of using multi-wavelength anomalous dispersion (MAD) in structure determination.
  • Using multi-wavelength anomalous dispersion (MAD) offers significant advantages for structure determination, as it allows researchers to gather phase information from multiple wavelengths that interact differently with heavy atoms in a crystal. This enhances accuracy and can lead to successful phasing where traditional methods might fail. However, MAD also has limitations such as the requirement for high-quality crystals and potential complications arising from radiation damage or variations in data quality across different wavelengths, which can affect the final model's reliability.

Related terms

Scattering factor: A measure of how much a specific atom in a crystal scatters X-rays, which is influenced by its electron density and the wavelength of the X-rays.

Phase problem: A challenge in crystallography where the phase information of scattered X-rays is lost, complicating the reconstruction of the electron density map of a crystal.

Multi-wavelength anomalous dispersion (MAD): A technique that uses multiple wavelengths of X-rays to exploit anomalous scattering from specific atoms, allowing for improved phase determination and structure elucidation.