<unk>  is the science of determining the arrangement of atoms within a crystal from the manner in which a beam of X-rays is scattered from the electrons within the crystal.   The method produces a three-dimensional picture of the density of electrons within the crystal, from which the mean atomic positions, their chemical bonds, their disorder and sundry other information can be derived.   By definition, a crystal is a solid in which a particular arrangement of atoms (its unit cell) is repeated indefinitely along  three principal directions known as the basis (or lattice) vectors, which are not necessarily perpendicular.  A wide variety of materials can form crystals — such as salts, metals, minerals, semiconductors, as well as various inorganic, organic and biological molecules — which has made<unk>  fundamental to many scientific fields.
The oldest and most precise method of<unk>  is single-crystal X-ray diffraction, in which a beam of X-rays is reflected from evenly spaced planes of a single crystal, producing a diffraction pattern of spots called reflections.  Each reflection corresponds to one set of evenly spaced planes within the crystal.  The density of electrons within the crystal is determined from the position and brightness of the various reflections observed as the crystal is gradually rotated in the X-ray beam; this density, together with supplementary data, allows the atomic positions to be inferred.  For single crystals of sufficient purity and regularity, X-ray diffraction data can determine the mean chemical bond lengths and angles to within a few thousandths of an Ångström and to within a few tenths of a degree, respectively.  The data also allow the static and dynamic disorder in the atomic positions to be estimated, which is usually less than a few tenths of an Ångström.  At its most basic level,<unk>  is useful in identifying known materials, characterizing new materials and in discerning materials that appear similar by other experiments.  However, X-ray crystal structures have many other applications; for example, they can account for unusual electronic or elastic properties of a material, shed light on chemical interactions and processes, or serve as the basis for understanding enzymatic mechanisms and designing pharmaceuticals against diseases.   The term<unk> " is also sometimes applied to methods that involve X-ray diffraction from polycrystalline materials, such as powders of small crystals studied by X-ray powder diffraction.
The technique of single-crystal<unk>  has three basic steps.  The first — and often most difficult — step is to obtain an adequate crystal of the material under study.  The crystal should be sufficiently large, pure in composition and regular in structure, with no large internal imperfections such as cracks or twinning.  In the second step, the crystal is placed in an intense beam of X-rays, usually of a single wavelength (monochromatic X-rays), producing the regular pattern of reflections.  As the crystal is gradually rotated, previous reflections disappear and new ones appear; the intensity of every spot is recorded meticulously at every orientation of the crystal.  Multiple data sets may have to be collected, with each set covering slightly more than half a full rotation of the crystal and containing tens of thousands of reflection intensities.   In the third step, these data are combined computationally with complementary chemical information to produce and refine a model of the arrangement of atoms within the crystal.  The final, refined model — now called a crystal structure — is usually stored in a public database.
As the crystal's unit cell becomes larger, the atomic-level picture provided by<unk>  becomes less well-resolved (more "fuzzy") for a given number of observed reflections.  Two limiting cases of<unk>  are often discerned, "small-molecule" and "macromolecular"<unk> .  Small-molecule<unk>  typically involves crystals with fewer than 100 atoms in their asymmetric unit; such crystal structures are usually so well resolved that its atoms can be discerned as isolated "blobs" of electron density.   By contrast, macromolecular<unk>  often involves tens of thousands of atoms in the unit cell.  Such crystal structures are generally less well-resolved (more "smeared out"); the atoms and chemical bonds appear as tubes of electron density, rather than as isolated atoms.  In general, small molecules are also easier to crystallize than macromolecules; however,<unk>  has proven possible even for viruses with hundreds of thousands of atoms.
X-ray crystallography
Electrophoresis
Fiber diffraction
Counterimmunoelectrophoresis
Enzyme assay
Biological small-angle X-ray scattering
Direct fluorescent antibody
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