Inchemistry, a Svedberg unitorsvedberg (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).[1] 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.[1]
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.
The unit is named after the Swedish chemist Theodor Svedberg (1884–1971), winner of the 1926 Nobel Prize[2]inchemistry for his work on disperse systems, colloids and his invention of the ultracentrifuge.[3]
The Svedberg coefficient is a nonlinear function.[1] 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.[1]
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 (26×10−13 s) will travel at 26 micrometers per second (26×10−6 m/s) under the influence of an acceleration of a million gravities (107 m/s2).[a] Centrifugal acceleration is given as rω2; 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.[1]
In centrifugation of small biochemical species, a convention has developed in which sedimentation coefficients are expressed in the Svedberg units.