Multi-wavelength anomalous dispersion

Introduction

Multi-wavelength anomalous dispersion (MAD) is a technique used in X-ray crystallography to solve the phase problem, a key issue in the interpretation of X-ray diffraction data. This method exploits the phenomenon of anomalous scattering, where the scattering of X-rays by an atom varies depending on the wavelength of the incident X-ray beam.

A laboratory setup showing an X-ray source, a crystal sample, and a detector.

Anomalous Scattering

Anomalous scattering occurs when an X-ray photon is absorbed by an atom and then re-emitted, causing a phase shift in the scattered wave. This effect is highly dependent on the energy of the incident X-ray, which can be tuned by changing its wavelength. The energy dependence of the anomalous scattering leads to a dispersion effect, which is the basis of the MAD method.

The Phase Problem

The phase problem arises because the detectors used in X-ray crystallography can only measure the intensity of the diffracted X-rays, not their phase. Without the phase information, it is impossible to reconstruct the electron density of the crystal, which is the ultimate goal of X-ray crystallography. MAD provides a way to estimate the missing phase information.

MAD Method

In the MAD method, X-ray diffraction data are collected at several different wavelengths near the absorption edge of a specific atom in the crystal. The differences in the anomalous scattering at these wavelengths allow the phase of the diffracted X-rays to be determined.

Applications

MAD has been widely used in the determination of protein structures, where it has proved particularly useful for solving the structures of large and complex proteins. It has also been used in the study of other types of crystals, including inorganic and organic compounds.

Advantages and Limitations

The main advantage of the MAD method is that it provides a direct way to solve the phase problem without the need for additional experimental data. However, it requires a crystal that contains an atom with a suitable absorption edge, and the data collection can be time-consuming and technically challenging.