Sedimentation (24.7)
•
When macromolecules move in solution, their motion is dictated by the
forces
they feel
–
Gravitational force (F
g
) is
opposed
by frictional (F
fr
) and buoyant (F
b
) forces
–
Specific volume of the solute is the change in solution volume upon addition of a given
amount of solvent (measure in cm
3
/g)
•
When all forces sum to zero, terminal velocity is achieved (v
ter
)
•
Sedimentation coefficient relates speed of sedimentation to properties of
macromolecule and solvent
–
Measured in units of Svedbergs (S) where 1S = 1 x 10

13
s
Velocity Sedimentation (24.7)
•
Velocity sedimentation utilizes centripetal force instead of gravity
–
ω is the angular velocity (i.e., speed of centrifuge) and x is the distance between the
particle and the center of rotation
•
Centrifugation
causes heavier particles (solute) to move away from center
of rotation
–
Boundary forms with one side being solution and the other side solvent
–
Sedimentation coefficient can be determined by monitoring motion of boundary
•
Sedimentation coefficients can give us molar mass (M = mN
A
) of the
macromolecule if we know the frictional coefficient
–
We can get this from a diffusion experiment
Sedimentation Forces
Centrifuge Cell for Velocity Sedimentation
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