In chemistry, a ****Svedberg unit or svedberg (symbol S, sometimes Sv) is a non-SI metric unit for sedimentation coefficients. The Svedberg unit offers a measure of a particle's size indirectly based on its sedimentation rate under acceleration (i.e. how fast a particle of given size and shape settles out of suspension). The svedberg is a measure of time, defined as exactly 10−13 seconds (100 fs). For biological macromolecules and cell organelles like ribosomes, the sedimentation rate is typically measured as the rate of travel in a centrifuge tube subjected to high g-force. The svedberg (S) is distinct from the SI unit sievert or the non-SI unit sverdrup, which also use the symbol Sv, and to the SI unit Siemens which uses the symbol S too.

Naming

The unit is named after the Swedish chemist Theodor Svedberg (1884–1971), winner of the 1926 Nobel Prize in chemistry for his work on disperse systems, colloids and his invention of the ultracentrifuge.

Factors

The Svedberg coefficient is a nonlinear function. A particle's mass, density, and shape will determine its S value. The S value depends on the frictional forces retarding its movement, which, in turn, are related to the average cross-sectional area of the particle. The sedimentation coefficient is the ratio of the speed of a substance in a centrifuge to its acceleration in comparable units. A substance with a sedimentation coefficient of 26S (26 s) will travel at 26 micrometers per second (26 m/s) under the influence of an acceleration of a million gravities (107 m/s2). Centrifugal acceleration is given as rω; where r is the radial distance from the rotation axis and ω is the angular velocity in radians per second. Bigger particles tend to sediment faster and so have higher Svedberg values. Svedberg units are not directly additive since they represent a rate of sedimentation, not weight.

Use

In centrifugation of small biochemical species, a convention has developed in which sedimentation coefficients are expressed in the Svedberg units.

Footnotes