Temperature value
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In crystallography, '''uncertainty''' in the positions of atoms increases with '''disorder''' in the protein crystal. Disorder may have two components, static and dynamic. First, some regions of the molecule may adopt different conformations in different copies of the molecule, each molecule's conformation being stable (static disorder). Second, some regions of every copy of the molecule may be subject to thermal motion, meaning vibration about the rest position<ref>[http://www.elsevier.com/wps/product/cws_home/707331 Rhodes, G. 2006. Crystallography Made Crystal Clear, 3rd ed. Academic Press.]</ref> Thermal motion is minimized when the crystal is frozen with liquid nitrogen while being irradiated, but irradiation may warm the crystal permitting some thermal motion. | In crystallography, '''uncertainty''' in the positions of atoms increases with '''disorder''' in the protein crystal. Disorder may have two components, static and dynamic. First, some regions of the molecule may adopt different conformations in different copies of the molecule, each molecule's conformation being stable (static disorder). Second, some regions of every copy of the molecule may be subject to thermal motion, meaning vibration about the rest position<ref>[http://www.elsevier.com/wps/product/cws_home/707331 Rhodes, G. 2006. Crystallography Made Crystal Clear, 3rd ed. Academic Press.]</ref> Thermal motion is minimized when the crystal is frozen with liquid nitrogen while being irradiated, but irradiation may warm the crystal permitting some thermal motion. | ||
| - | Some regions of the molecule may have higher average disorder, and others lower average disorder. Typically, the ends of chains have higher average disorder, and hence their positions are less certain than are residues in the core of a tightly packed domain, where disorder is less. | + | Some regions of the molecule may have higher average disorder, and others lower average disorder. Typically, the ends of chains have higher average disorder, and hence their positions are less certain than are residues in the core of a tightly packed domain, where disorder is less. The disorder for each atom is quantitated in its ''temperature value''. |
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| + | Visualizing relative disorder or uncertainty in atomic positions is done by ''coloring by temperature value''. Atoms with <font color='blue'>'''low temperature values are colored blue'''</font>, while atoms with <font color='red'>'''high temperature values are colored red'''</font>. | ||
Often the very ends of chains, or surface loops, may be so disordered as to prevent assigning any atomic positions at all, leading to '''missing residues'''. ''FirstGlance in Jmol'' (linked beneath the molecule on every [[PDB code]]-titled page in Proteopedia) has a '''Gaps''' button that explains how to detect and visualize missing residues. | Often the very ends of chains, or surface loops, may be so disordered as to prevent assigning any atomic positions at all, leading to '''missing residues'''. ''FirstGlance in Jmol'' (linked beneath the molecule on every [[PDB code]]-titled page in Proteopedia) has a '''Gaps''' button that explains how to detect and visualize missing residues. | ||
Revision as of 00:42, 27 June 2008
In crystallography, uncertainty in the positions of atoms increases with disorder in the protein crystal. Disorder may have two components, static and dynamic. First, some regions of the molecule may adopt different conformations in different copies of the molecule, each molecule's conformation being stable (static disorder). Second, some regions of every copy of the molecule may be subject to thermal motion, meaning vibration about the rest position[1] Thermal motion is minimized when the crystal is frozen with liquid nitrogen while being irradiated, but irradiation may warm the crystal permitting some thermal motion.
Some regions of the molecule may have higher average disorder, and others lower average disorder. Typically, the ends of chains have higher average disorder, and hence their positions are less certain than are residues in the core of a tightly packed domain, where disorder is less. The disorder for each atom is quantitated in its temperature value.
Visualizing relative disorder or uncertainty in atomic positions is done by coloring by temperature value. Atoms with low temperature values are colored blue, while atoms with high temperature values are colored red.
Often the very ends of chains, or surface loops, may be so disordered as to prevent assigning any atomic positions at all, leading to missing residues. FirstGlance in Jmol (linked beneath the molecule on every PDB code-titled page in Proteopedia) has a Gaps button that explains how to detect and visualize missing residues.
In the PDB file format, each atom is given not only X, Y, and Z Cartesian coordinates, but two additional values immediately following called occupancy and temperature value (also known as the isotropic B value). If the end of a chain adopts either of two stable positions with equal probability, each position has 50% occupancy. The temperature factor is provided to quantitate the level of thermal motion. However, these two components of disorder cannot be distinguished with crystal diffraction data alone. Therefore, the occupancy is often given as 1.0 (100%), while the degree of "blur" in the electron density map, representing both components of disorder, is reported in the temperature value field. These values are mapped to colors when a crystallographic result is colored by temperature.
NMR models provide no information in the temperature value fields of PDB files. Rather, the variation between the models gives some indication of uncertainty or flexibility.
