- Sedimentation coefficient
The sedimentation coefficient "s" of a particle is used to characterize its behaviour in
sedimentation processes, notablycentrifugation . It is defined as the ratio of a particle's sedimentationvelocity to theacceleration 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 theviscous resistance of the medium (normallywater ) through which the particle is moving. The applied acceleration "a" (in ms−2) can be either thegravitational acceleration "g", or more commonly thecentrifugal acceleration m omega^2 r. In the latter case "m" is the mass of a particle, omega is theangular velocity of therotor and "r" is the distance of a particle to the rotor axis (radius ).The sedimentation coefficient has the
dimension s of a unit oftime 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 pullingforce 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 frombacteria actually has a sedimentation coefficient of 70 svedberg. It is composed of a50 S subunit and a30 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]
Wikimedia Foundation. 2010.