Sedimentation Coefficient Calculator

Calculate sedimentation coefficient S in Svedbergs from ultracentrifugation boundary velocity.

30000 RPM = 3141 rad/s
Proteins: 0.73
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Why Svedberg Values Do Not Add

The sedimentation coefficient depends on both mass and shape. A larger particle sediments faster, but so does a more compact one — and assembling two subunits changes the shape as well as the mass.

s = v / (ω²r)     1 S = 10−13 seconds

The classic illustration is the bacterial ribosome: a 30S subunit plus a 50S subunit gives a 70S particle, not 80S. The assembled complex is more compact than the two separately, so it sediments proportionally faster than mass alone predicts — but not by the arithmetic sum.

ParticleS valueNote
Bacterial small subunit30S—
Bacterial large subunit50S—
Bacterial ribosome70SNot 80S
Eukaryotic ribosome80S40S + 60S
IgG antibody7S—
Catalase11S—

Shape as Well as Size

Two particles of identical mass can have very different S values if their shapes differ. A compact globular protein sediments faster than an extended or fibrous one of the same molecular weight, because it experiences less frictional drag.

This makes sedimentation useful for detecting conformational change: a protein that unfolds or elongates shows a reduced S value at unchanged mass. The frictional ratio f/f0, derived from combining sedimentation with diffusion data, quantifies how far a particle departs from a sphere.

The Svedberg equation combines both measurements to give molecular weight directly, which is why analytical ultracentrifugation remains a reference method for characterising protein complexes and their stoichiometry in solution.

Worked Examples

Example 1: Small ribosomal subunit boundary: v=5e-5cm/s, omega=3141, r=6.5
S=5e-5/(3141^2x6.5)x10^13
Result: S≈7.8S — close to small subunit
Boundary velocity depends on conditions
Example 2: IgG antibody 7S
v=omega^2 x r x 7e-13
Result: Calculate expected velocity
Measure at known RPM and radius
Example 3: Why 30S + 50S = 70S
Two subunits assembling into a ribosome
Result: Not 80S
Association buries surface area and produces a more compact particle. Sedimentation reflects the assembled hydrodynamic properties, not the sum of the parts.
Example 4: Detecting unfolding
Protein S value falls with unchanged mass
Result: Conformational change
An extended conformation experiences more frictional drag and sediments more slowly. This is a standard way to detect denaturation in solution.

Common Mistakes

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Adding Svedberg values of subunits

They are not additive. A 30S and 50S subunit assemble into a 70S ribosome because the complex is more compact than its parts.

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Treating S as a pure size measure

Shape matters as much. An elongated protein sediments more slowly than a globular one of identical mass.

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Forgetting solvent conditions

S values depend on temperature, buffer density and viscosity. Published values are normally corrected to water at 20°C, written s20,w.

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Assuming S alone gives molecular weight

The Svedberg equation also requires the diffusion coefficient. Sedimentation alone cannot separate mass from shape.

Frequently Asked Questions

Svedberg equation for MW?
M=RTs/[D(1-v_bar*rho)]. Requires both sedimentation coefficient s AND diffusion coefficient D. Without D: only qualitative size estimation from S value.
Analytical ultracentrifugation?
AUC tracks sedimentation in real time. Sedimentation velocity: s-value distribution. Equilibrium: MW distribution. Gold standard for macromolecular assembly without labeling.
Why don't Svedberg values add?
Because sedimentation depends on shape as well as mass. An assembled complex is more compact than its separated subunits, so 30S + 50S gives 70S.
What does the Svedberg unit measure?
Sedimentation rate per unit centrifugal field, with 1 S equal to 10−13 seconds. Larger and more compact particles have higher values.
Can I get molecular weight from S alone?
No. The Svedberg equation also needs the diffusion coefficient, since sedimentation alone cannot distinguish a heavy elongated particle from a lighter compact one.
What is s20,w?
A sedimentation coefficient corrected to water at 20°C, allowing values measured in different buffers and temperatures to be compared.