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| 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). | | The name of the writer is Jayson. Office supervising is where her main earnings arrives from but she's already utilized for an additional one. To perform lacross is the factor I love psychics ([http://www.publicpledge.com/blogs/post/7034 www.publicpledge.com]) most of all. Alaska is the only location I've been residing in but now I'm considering other options. |
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| : <math>s = \frac{v_t}{a}</math>
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| The sedimentation speed <math>v_t</math> (in [[metre per second|ms<sup>−1</sup>]]) 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 tens of thousands of [[standard gravity|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 [[metre per second squared|ms<sup>−2</sup>]]) can be either the [[gravitational acceleration]] ''g'', or more commonly the [[centrifugal acceleration]] <math>\omega^2 r</math>. In the latter case, <math>\omega</math> is the [[angular velocity]] of the [[Turbine|rotor]] and ''r'' is the distance of a particle to the rotor axis ([[radius]]).
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| The viscous resistance is given by the Stokes' law: 6πηr<sub>0</sub>v where η is the viscosity of the medium, r<sub>0</sub> is the radius of the particle and v is the velocity of the particle. This law applies only for large spheres in an infinite sea of the fluid.
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| The centrifugal force is given by the familiar equation: mrω<sup>2</sup>. Here r is the distance of the particle from the axis of rotation. When the two forces (viscous force and the centrifugal force) balance (they are oppositely directed), the particle moves with constant velocity called the terminal velocity. Hence the terminal velocity is given by the following equation.
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| : <math>{v_t} = \frac{mr\omega^2}{6\pi \eta r_0}</math>
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| Rearranging this equation we get the final formula:
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| : <math>s = \frac{v_t}{r\omega^2} = \frac{m}{6\pi \eta r_0} </math>
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| The sedimentation coefficient has the [[dimension]]s of a unit of [[time]] and is expressed in [[svedberg]]s. One svedberg is defined as exactly 10<sup>−13</sup> [[second|s]]. Essentially the sedimentation coefficient serves to normalize the sedimentation rate of a particle by the acceleration 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.
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| Bigger particles sediment faster and have higher sedimentation coefficients (svedberg, or S values). Sedimentation coefficients are, however, not additive. Sedimentation rate does not depend only on the mass or volume of a particle, and when two particles bind together there is inevitably a loss of surface area. Thus when measured separately they will have svedberg values that may not add up to that of the bound particle. This is notably the case with the [[ribosome]]. Ribosomes are most often identified by their sedimentation coefficient. For instance, the 70 S ribosome that comes from [[bacteria]] has actually a sedimentation coefficient of 70 svedberg, although it is composed of a [[50 S]] subunit and a [[30 S]] subunit.
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| ==See also==
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| * [[Clearing factor]]
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| * [[Svedberg]]
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| * [[Sedimentation]]
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| * [[Centrifugation]]
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| ==External links==
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| * [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]
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| * [http://www.astbury.leeds.ac.uk/facil/AUC/Vel_data_analysis.htm Analysis of sedimentation velocity data]
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| * [http://www.ap-lab.com/sedimentation_velocity.htm An article on sedimentation velocities] on the Alliance Protein Laboratories website
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| * [http://www.proteinscience.org/cgi/content/full/11/9/2067 Modern Analytical Ultracentrifugation in Protein Science: A tutorial review]
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| <!--Categories-->
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| [[Category:Laboratory techniques]]
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| [[Category:Unit operations]]
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