Xray diffraction definition11/23/2023 The resolution of a diffraction pattern depends on how ordered the crystal is. Perhaps the greatest challenge facing crystallographers is to obtain a well-ordered crystal that diffracts to "atomic resolution". More than half of the crystals obtained from various purified proteins are not of "atomic resolution", that is, the disorder it too great to permit determination of molecular structure. Resolution is theoretically limited by the wave length of X-rays (on the order of 1 Å), but in practice, the quality of the available crystals determines resolution. In crystals of macromolecules, it is the degree of order in the crystal ("quality of the crystal") that limits the resolution of X-ray crystallography. For non-experts, it would be less confusing if the terms were fine and rough resolution rather than high and low resolution, but high and low are the established terms in the field. For example, a 1.0 Å structure resolves finer detail than a 4.0 Å structure, so the 1.0 Å structure is said to have higher resolution than the 4.0 Å structure. High resolution is characterized by being able to distinguish smaller features, so there is an inverse relationship between the quality of a structure and the length value given for the resolution. The temperature value for each atom in a crystallographic model indicates the clarity of its electron density map. Atoms unclear.īear in mind that the resolution value represents the "best" (most ordered) parts of a crystal, and that other parts may have poorer quality. ~8.0 Å: Largest features seen roughly. Here is a quick guide for interpreting values of resolution (taken from FirstGlance in Jmol). These single-atom uncertainties are called the B factors or temperature values of the atoms (see Temperature). The "resolution of the crystal" represents the most ordered portions (see Determination of Resolution).Īfter an electron density map is calculated and refined with a fitted atomic model, an uncertainty of atomic position is calculated for each atom in the model. If some portions of the macromolecule are less ordered in the crystal than others, these will have a poorer resolution. For comparison, the van der Waals diameter of a carbon atom is 3.4-3.7 Å, and the length of a covalent carbon-carbon bond is 1.5 Å. High numeric values of resolution, such as 4 Å, mean poor resolution, while low numeric values, such as 1.5 Å, mean good resolution.Ģ.05 Å is the median resolution for X-ray crystallographic results in the Protein Data Bank (135,762 on May 19, 2019). In X-ray crystallography, resolution is the smallest distance between crystal lattice planes that is resolved in the diffraction pattern. The atomic model, shown as sticks, is then built, guided by the electron density map. Structure determination by X-ray crystallography or cryo-electron microscopy produces an electron density map (shown in green). Resolution, in structure determinations, is the distance corresponding to the smallest observable feature: if two objects are closer than this distance, they appear as one combined blob rather than two separate objects. Snapshots from the movie (see below) by James Holton. Jump to: navigation, search proteopedia link proteopedia link
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