Svedberg

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For the Swedish ice hockey player, see Viktor Svedberg.

A laboratory ultracentrifuge.

A svedberg (symbol S, sometimes Sv, not to be confused with Sv for the SI unit sievert as well as the non-SI sverdrup) is a non-SI physical unit used for sedimentation coefficients.  It characterizes the behaviour of a particle type in sedimentation processes, notably centrifugation.  The svedberg is technically a measure of time, and is defined as exactly 10^-13 seconds (100 fs).

Conventionally all sedimentation coefficients are expressed in the Svedberg units (i.e., 100 fs).  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 (26x10^-13 s) will travel at 26 microns per second (26x10^-6 m/s) under the influence of an acceleration of a million gravities (10⁷ m/s²).  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.

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

Bigger particles tend to sediment faster and thus have higher svedberg 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.

The svedberg is the most important measure used to distinguish ribosomes.

See also

• Sedimentation coefficient

External links

• Svedberg unit - Nobelprize.org