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Sedimentation Coefficient – Definition and Importance

The sedimentation coefficient describes how fast a particle sediments in a centrifuge. It is measured in Svedberg units (S) and is a key parameter in biochemistry and medicine.

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Things worth knowing about "Sedimentation Coefficient"

The sedimentation coefficient describes how fast a particle sediments in a centrifuge. It is measured in Svedberg units (S) and is a key parameter in biochemistry and medicine.

What is the Sedimentation Coefficient?

The sedimentation coefficient (also known as the Svedberg constant or S-value) is a physicochemical parameter that describes how rapidly a particle -- such as a protein, cell organelle, or virus -- migrates through a liquid under the influence of centrifugal force. It is named after the Swedish chemist Theodor Svedberg, who was awarded the Nobel Prize in Chemistry in 1926 for his pioneering work on ultracentrifugation.

The unit of the sedimentation coefficient is the Svedberg (S), defined as: 1 S = 10^-13 seconds. Typical biological macromolecules have S-values ranging from 1 S to several hundred S.

Physical Principles

The sedimentation coefficient s is defined as the ratio of the sedimentation velocity of a particle to the applied centrifugal acceleration:

Sedimentation velocity: How fast does the particle move toward the bottom of the centrifuge tube?
Centrifugal acceleration: How strong is the outward force, expressed as a multiple of gravitational acceleration (g)?

Key factors influencing the S-value include the mass of the particle, its density, its shape, and the viscosity and density of the surrounding solvent. Larger and heavier particles generally have higher S-values.

Importance in Biochemistry and Medicine

The sedimentation coefficient is a fundamental tool in modern biochemistry, cell biology, and medicine. It allows biological macromolecules and cellular components to be characterized and separated. Well-known examples include:

Ribosomes: The ribosomal subunits of prokaryotes are designated 30S and 50S, with the complete ribosome having an S-value of 70S. In eukaryotes, the subunits are 40S and 60S (80S ribosome). This nomenclature is fundamental to understanding protein synthesis and to developing antibiotics that selectively target ribosomal subunits.
Proteins and enzyme complexes: The S-value helps determine the size and composition of protein complexes.
Viruses: Viruses are characterized by their S-value, which is relevant in virology and diagnostics.
Cell organelles: Mitochondria, lysosomes, and other organelles can be separated by density gradient centrifugation based on their S-values.

Measurement: Ultracentrifugation

The sedimentation coefficient is determined using the ultracentrifuge. In analytical ultracentrifugation, the sample is subjected to very high centrifugal forces inside a rotating cell, and the migration of the particles is monitored optically. Preparative ultracentrifugation is used to physically separate particles for further analysis.

Sedimentation Coefficient vs. Molecular Mass

Although the S-value is related to the mass of a particle, it is not a direct measure of it. Two particles with the same mass can have different S-values if they differ in shape or density. For precise mass determination, complementary methods such as mass spectrometry or gel permeation chromatography are used.

Clinical Relevance

In clinical diagnostics and research, the sedimentation coefficient is significant for:

Characterizing pathogens such as viruses and bacteria
Developing antibiotics that inhibit specific ribosomal subunits (e.g., 30S or 50S)
Analyzing lipoproteins (e.g., LDL, HDL) in the blood, which are separated by ultracentrifugation
Basic research on protein misfolding diseases such as Alzheimer disease or Parkinson disease, where protein aggregates are studied

References

Lodish H. et al. - Molecular Cell Biology, 8th Edition, W. H. Freeman and Company, 2016.
Berg J. M., Tymoczko J. L., Stryer L. - Biochemistry, 8th Edition, W. H. Freeman and Company, 2015.
Svedberg T., Pedersen K. O. - The Ultracentrifuge, Oxford University Press, 1940.

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