Sedimentation coefficient

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Sedimentation coefficient

The sedimentation coefficient "s" of a particle is used to characterize its behaviour in sedimentation processes, notably centrifugation. It is defined as the ratio of a particle's sedimentation velocity to the acceleration that is applied to it (causing the sedimentation).

: $s = frac\left\{v_t\right\}\left\{a\right\}$

The sedimentation speed $v_t$ (in ms−1) is also known as the terminal velocity. It is constant because the force applied to a particle by gravity or by a centrifuge (measuring typically in multiples of hundreds of thousands of gravities in an ultracentrifuge) is cancelled by the viscous resistance of the medium (normally water) through which the particle is moving. The applied acceleration "a" (in ms−2) can be either the gravitational acceleration "g", or more commonly the centrifugal acceleration $m omega^2 r$. In the latter case "m" is the mass of a particle, $omega$ is the angular velocity of the rotor and "r" is the distance of a particle to the rotor axis (radius).

The sedimentation coefficient has the dimensions of a unit of time and is expressed in svedbergs. One svedberg is defined as exactly 10−13 s. Essentially the sedimentation coefficient serves to normalize the sedimentation rate of a particle to the pulling force applied to it. The resulting value is no longer dependent on the acceleration, but depends only on the properties of the particle and the medium in which it is suspended. Sedimentation coefficients quoted in literature usually pertain to sedimentation in water at 20°C.

Bigger particles tend to sediment faster, and thus have higher svedberg values. Sedimentation coefficients are however not additive. Since the sedimentation rate is associated with (among other factors) the size of the particle, when two particles bind together there is inevitably a loss of surface area. Thus when measured separately they will have svedberg values that do not add up to that of the particle formed when they bind together.

This is particularly the case with the ribosome. The most important measure used to distinguish ribosomes is the svedberg. For instance, the 70 S ribosome that comes from bacteria actually has a sedimentation coefficient of 70 svedberg. It is composed of a 50 S subunit and a 30 S subunit.

ee Also

* Clearing factor
* Svedberg
* Sedimentation
* Centrifugation

* [http://homepages.gac.edu/~cellab/chpts/chpt3/intro3.html Introduction to Cell Fractionation] in the [http://homepages.gac.edu/~cellab/index-1.html Cell Biology Laboratory Manual]
* [http://www.astbury.leeds.ac.uk/facil/AUC/Vel_data_analysis.htm Analysis of sedimentation velocity data]
* [http://www.ap-lab.com/sedimentation_velocity.htm An article on sedimentation velocities] on the Alliance Protein Laboratories website
* [http://www.proteinscience.org/cgi/content/full/11/9/2067 Modern Analytical Ultracentrifugation in Protein Science: A tutorial review]

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Look at other dictionaries:

• sedimentation coefficient — n a measure of the rate at which a molecule (as a protein) suspended in a colloidal solution sediments in an ultracentrifuge usu. expressed in svedbergs * * * the velocity at which a particle sediments in a centrifuge divided by the applied… …   Medical dictionary

• sedimentation coefficient — sedimentation coefficient. См. коэффициент седиментации. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

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• sedimentation coefficient — sedimentacijos koeficientas statusas T sritis chemija apibrėžtis Koeficientas, apibūdinantis dalelių nusėdimo greitį centrifuguojant. atitikmenys: angl. sedimentation coefficient rus. коэффициент седиментации …   Chemijos terminų aiškinamasis žodynas

• sedimentation coefficient — sedimentacijos koeficientas statusas T sritis augalininkystė apibrėžtis Rodiklis, rodantis mikrodalelių nusėdimo greitį. atitikmenys: angl. sedimentation coefficient rus. коэффициент седиментации …   Žemės ūkio augalų selekcijos ir sėklininkystės terminų žodynas

• sedimentation coefficient — The ratio of the velocity of sedimentation of a molecule to the centrifugal force required to produce this sedimentation. It is a constant for a particular species of molecule, and the value is given in Svedberg units (S) that, it should be noted …   Dictionary of molecular biology

• sedimentation coefficient — noun : a measure of the rate at which a molecule (as a protein) suspended in a colloidal solution sediments in an ultracentrifuge usually expressed in svedbergs …   Useful english dictionary

• Sedimentation — describes the motion of molecules in solutions or particles in suspensions in response to an external force such as gravity, centrifugal force or electric force. Sedimentation may pertain to objects of various sizes, ranging from suspensions of… …   Wikipedia

• coefficient de sédimentation — sedimentacijos koeficientas statusas T sritis Standartizacija ir metrologija apibrėžtis Koeficientas, apibūdinantis dalelių nusėdimo spartą centrifuguojant. atitikmenys: angl. sedimentation coefficient vok. Sedimentationskoeffizient, m rus.… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

• sédimentation — [ sedimɑ̃tasjɔ̃ ] n. f. • 1861; de sédiment 1 ♦ Méd. Formation de sédiment. Sédimentation sanguine : dépôt des globules rouges du sang rendu incoagulable, au fond du tube où il est laissé en repos. Vitesse de sédimentation, dont la détermination… …   Encyclopédie Universelle