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A powder X-ray diffractometer in motion. X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract in specific directions.
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] The phase problem is also met in the fields of imaging and signal processing. [2] Various approaches of phase retrieval have been developed over the years.
Crystallography is a broad topic, and many of its subareas, such as X-ray crystallography, are themselves important scientific topics. Crystallography ranges from the fundamentals of crystal structure to the mathematics of crystal geometry, including those that are not periodic or quasicrystals.
As the crystal's repeating unit, its unit cell, becomes larger and more complex, the atomic-level picture provided by X-ray crystallography becomes less well-resolved (more "fuzzy") for a given number of observed reflections. Two limiting cases of X-ray crystallography are often discerned, "small-molecule" and "macromolecular" crystallography.
Molecular replacement (MR) [1] is a method of solving the phase problem in X-ray crystallography.MR relies upon the existence of a previously solved protein structure which is similar to our unknown structure from which the diffraction data is derived.
Isomorphous replacement (IR) is historically the most common approach to solving the phase problem in X-ray crystallography studies of proteins.For protein crystals this method is conducted by soaking the crystal of a sample to be analyzed with a heavy atom solution or co-crystallization with the heavy atom.