World Library  
Flag as Inappropriate
Email this Article

Sedimentation coefficient

Article Id: WHEBN0003402094
Reproduction Date:

Title: Sedimentation coefficient  
Author: World Heritage Encyclopedia
Language: English
Subject: Unit operations, Differential centrifugation, Svedberg, Laboratory techniques
Collection: Laboratory Techniques, Unit Operations
Publisher: World Heritage Encyclopedia
Publication
Date:
 

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 tens 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 \omega^2 r. In the latter case, \omega is the angular velocity of the rotor and r is the distance of a particle to the rotor axis (radius).

The viscous resistance is given by the Stokes' law: 6πηr0v where η is the viscosity of the medium, r0 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.

The centrifugal force is given by the familiar equation: mrω2. 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.

{v_t} = \frac{mr\omega^2}{6\pi \eta r_0}

Rearranging this equation we get the final formula:

s = \frac{v_t}{r\omega^2} = \frac{m}{6\pi \eta r_0}

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 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.

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.

See also

External links

  • Introduction to Cell Fractionation in the Cell Biology Laboratory Manual
  • Analysis of sedimentation velocity data
  • An article on sedimentation velocities on the Alliance Protein Laboratories website
  • Modern Analytical Ultracentrifugation in Protein Science: A tutorial review
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.