S (Svedberg unit)

Svedberg unit (S) is a non-SI unit for sedimentation rate. The sedimentation rate is used to measure the rate at which particles suspended in a liquid settle out of the fluid through a process known as centrifugation. This measurement is significant in the study of macromolecules, cell organelles, and other particles in biochemistry and molecular biology. The Svedberg unit is named after the Swedish chemist Theodor Svedberg (1884–1971), who was awarded the Nobel Prize in Chemistry in 1926 for his work on disperse systems.

Definition

The Svedberg unit is a measure of time, specifically sedimentation rate, defined as 10^-13 seconds (10 picoseconds). However, it is not a measure of time in the conventional sense but rather a measure of sedimentation velocity. The sedimentation velocity is influenced by several factors, including the size, shape, and density of the particles, as well as the density and viscosity of the medium in which they are suspended.

Measurement

Sedimentation rates are measured using an ultracentrifuge, which subjects the particles to a high centrifugal force, allowing for the separation of molecules of different sizes or densities. The sedimentation rate of a particle in an ultracentrifuge is given by the equation:

\[S = \frac{v}{ω^2r}\]

where v is the sedimentation velocity, ω is the angular velocity, and r is the radial distance from the axis of rotation. The unit of S is typically expressed in Svedberg units (S), which helps in comparing the sedimentation rates of different particles.

Applications

The Svedberg unit is widely used in the field of molecular biology, biochemistry, and genetics for the separation and analysis of biomolecules, including DNA, RNA, proteins, and ribosomes. For example, ribosomes are often referred to by their sedimentation rates, with prokaryotic ribosomes being 70S and eukaryotic ribosomes 80S, where the "S" stands for Svedberg units. This differentiation is crucial for the study of protein synthesis and the action of antibiotics.

Limitations

While the Svedberg unit is useful for comparing the sedimentation rates of particles, it is important to note that sedimentation rates are not additive. This means that a particle composed of two subunits, each with a sedimentation rate of 10S, will not necessarily have a combined sedimentation rate of 20S. This is due to the complex relationship between sedimentation rate and the shape, size, and interaction of the particles.

See Also

• Centrifugation
• Ultracentrifuge
• Biochemistry
• Molecular biology
• Theodor Svedberg

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