1. sedimentaion

2. Sedimentation
Sedimentation describes the motion of molecules in solutions or particles in suspensions in response to an external force such as gravity, centrifugal force or electric force.

3. Sedimentation Theory Svedberg equations
Centrifugal force = buoyant force + frictional force

4. 1. The Centrifugal Force Fc = M * w * r2
Mo is the particle weight, or molecular weight
w (omega)= angular velocity (radians/sec)
r is the radius of rotation
This equation says that the larger the molecule, or the faster the centrifugation, or the longer the axis of rotation, the greater the centrifugal force and the rate of sedimentation.

5. The Centrifugal Force
A more common expression is the relative centrifugal force (RCF):
r = Radial distance of particle from axis of rotation
rpm = Revolutions ( round) per minute
RCF reports centrifugal force relative to earth’s gravitational force; commonly refer to as “number of times or g.”
A sample rotating at 20, 000 rpm with r = 7 cm will experience RCF= 33,000 x g.

6. 2.The Buoyant Force FB = Mw * w * r
The buoyant force opposes the centrifugal force.
where Mw is the mass of the solvent displaced by the particle.
The net force= (Fc-FB) will determine whether a particle floats or sediments
Particles with higher density will experience smaller buoyant force, and thus, sediment faster.

7. Frictional force (resistance of a molecule to movement)
The Frictional force
Ff = f V
Frictional force (resistance of a molecule to movement)
v = velocity relative to the centrifuge tube,
f = frictional coefficient.

8. The frictional coefficient depends upon:
the size
shape of the molecule,
the viscosity of the gradient material.
The frictional coefficient f of a compact particle is smaller than that of an extended particle of the same mass.

9. Centrifuge
Centrifuge is a piece of equipment, generally driven by a motor, that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis. The centrifuge works using the sedimentation principle

10. Applied Centrifugation
Parameters you need to know:
1. Type of rotor:
fixed angle, swinging bucket, vertical
2. Type of centrifuge:
Low speed , high Speed, ultracentrifuge
3. Type of centrifugation
Differential, preparative, or analytical
Also, the Speed and duration of centrifugation

11. 1. Types of Rotors
Dr Gihan Gawish

12. 2. Type of Centrifuge 2-1.Low-speed centrifuges
Also called: microfuge, Clinical, Table top or bench top centrifuges
Max speed ~ 20,000 rpm
Operate at room temperature
Fixed angle or swinging bucket can be used
Commonly used for rapid separation of coarse particles
E.g. RBC from blood, DNA from proteins, etc.
The sample is centrifuged until the particles are tightly packed into pellet at the bottom of the tube. Liquid portion, supernatant, is decanted.

13. 2-2. High-speed Centrifuges Preparative centrifuges
Max speed ~ 80,000 rpm
Often refrigerated, and requires vacuum to operate
Fixed angle or swinging bucket can be used
Generally used to separate macromolecules (proteins or nucleic acids) during purification or preparative work.
Can be used to estimate sedimentation coefficient and MW

14. 2-3. Ultracentrifuge
The most advanced form of centrifuges: (specialized and expensive)
Used to precisely determine sedimentation coefficient and MW of molecules, Molecular shape,Protein-protein interactions
Uses very high speed
Uses small sample size (< 1 ml)
Uses relatively pure sample
Built in optical system to analyze movements of molecules during centrifugation
Analytical Ultracentrifuge

15. 3. Types of Centrifugation
There are basically three modes of centrifugation
3-1.Differential or pelleting
Cellular fractionation and/or separating coarse suspension
removal of precipitates
crude purification step

16. 3-2. Preparative or Density gradient centrifugation:
Separation of complex mixtures
Finer fractionation of cellular components
Purification of proteins, nucleic acids, plasmids
Characterization of molecular interactions

17. 3-3. Analytical Relative MW Molecular shape Aggregation behavior
Determining hydrodynamic or thermodynamic properties of bio molecules
Relative MW
Molecular shape
Aggregation behavior
Protein-protein interactions