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).

: $s\; =\; frac\{v\_t\}\{a\}$

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

External links

* [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]