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In physics, the phase problem is the problem of loss of information concerning the phase that can occur when making a physical measurement. The name comes from the field of X-ray crystallography , where the phase problem has to be solved for the determination of a structure from diffraction data. [ 1 ]
An X-ray diffraction pattern of a crystallized enzyme. The pattern of spots (reflections) and the relative strength of each spot (intensities) can be used to determine the structure of the enzyme. The relative intensities of the reflections provides information to determine the arrangement of molecules within the crystal in atomic detail.
X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering , when there is no change in the energy of the waves.
Today, selenium-SAD is commonly used for experimental phasing due to the development of methods for selenomethionine incorporation into recombinant proteins. SAD is sometimes called "single-wavelength anomalous dispersion" , but no dispersive differences are used in this technique since the data are collected at a single wavelength.
Multi-wavelength anomalous diffraction (sometimes Multi-wavelength anomalous dispersion; abbreviated MAD) is a technique used in X-ray crystallography that facilitates the determination of the three-dimensional structure of biological macromolecules (e.g. DNA, drug receptors) via solution of the phase problem. [1]
RHEED is used to interrogate surface structure. [1] [2] Surface X-ray diffraction (SXRD), which is similar to RHEED but uses X-rays, and is also used to interrogate surface structure. [3] X-ray standing waves, another X-ray variant where the intensity decay into a sample from diffraction is used to analyze chemistry. [4]
Note that X-ray diffraction is sometimes considered a sub-set of X-ray scattering, where the scattering is elastic and the scattering object is crystalline, so that the resulting pattern contains sharp spots analyzed by X-ray crystallography (as in the Figure).
Anomalous X-ray scattering (AXRS or XRAS) is a non-destructive determination technique within X-ray diffraction that makes use of the anomalous dispersion that occurs when a wavelength is selected that is in the vicinity of an absorption edge of one of the constituent elements of the sample. It is used in materials research to study nanometer ...