Centrifugation sedimentation coefficients

The ribosome is the structure on which translation occurs. Figure 27.13 shows that the prokaryotic ribosome consists of two large subunits. Each subunit, in turn, consists of multiple proteins and RNA. The VOS, 508, and 308 designations refer to centrifugation sedimentation coefficients, which reflect the size of each entity, but are not strictly additive. 

Centrifugation methods separate macromolecules on the basis of their characteristic densities. Particles tend to fail through a solution if the density of the solution is less than the density of the particle. The velocity of the particle through the medium is proportional to the difference in density between the particle and the solution. The tendency of any particle to move through a solution under centrifugal force is given by the sedimentation coefficient, S ... 

Sedimentation coefficients are a measure of the velocity witli which a particle sediments in a centrifugal force field. Sedimentation coefficients are typically expressed in Svedbergs (.symbolized S), named to honor The Svedberg, developer of the ultracentrifuge. One S equals 10 . sec. 

It is convenient to classify particles by their rate of sedimentation per unit centrifugal field, a parameter known as the sedimentation coefficient (s), which has units of seconds. A Svedberg unit (S) is defined as a sedimentation coefficient of 1 X 10 13 seconds. 

The sedimentation coefficient may be determined by measuring the velocity of the particle at a fixed centrifuge speed (o>, in radians per second) and, from a series of observations, plotting the logarithm of the distance moved (x) against the time taken in seconds (t). The relationship is expressed by the equation ... 

D=mass diffumsion coefficient Z)T=fiiermal diffusion coefficient /=friction coefficient G=(oh (centrifugal acceleration) / =Boltzniann constant meff=particle effective mass r=radius of centrifuge basket s=sedimentation coefficient T = absolute temperature =geometric volume of die channel w=channel thickness y=diermal expansion coefficient p=electrophoretic... 

Figure 3. Concentration of poly(l-amidoethylene) in a centrifuged sample plotted as a function of sedimentation coefficient.

The results of a sedimentation experiment in a centrifugal field are conventionally reported in terms of what is known as a sedimentation coefficient. This quantity is defined as... 

The sedimentation coefficient depends on concentration consequently, it is usually measured at several different concentrations, and the results are extrapolated to zero concentration. It is customary to designate this limiting value by a superscript zero. Experimental values are also generally labeled with respect to temperature, so a value listed as 520 corresponds to a sedimentation coefficient measured at (or corrected to) 20°C and extrapolated to zero concentration. Under stationary state conditions, the force due to the centrifugal field and the viscous force of resistance will be equal. Therefore, co2r replaces g in Equation (4) to give... 

Extensive application of the theory has been made in treating pressure effects on velocity centrifugation. If w denotes angular speed, sp the sedimentation coefficient at pressure p, ra the radius of the meniscus, x = (r/ra)2, the usual representation of pressure effects leads to the relation (10),... 

It is cumbersome and sometimes impractical to express sedimentation velocity in terms of p, v, and/ since these factors are difficult to measure. A new term, sedimentation coefficient, s (the ratio of sedimentation velocity to centrifugal force) is introduced by rearranging Equation 7.7 to Equation 7.8. 

The sedimentation coefficient of spleen exonuclease, measured by centrifugation in a sucrose density gradient, using cytochrome c as the reference protein, was 4.6 S 11). The enzyme is eluted from Sephadex G-100 between acid phosphomonoesterase (s = 5.6 S) and acid DNase (s = 3.4 S). 

Sedimentation. The sedimentation method, described previously 5), is based on the relative sedimentation rates of swollen and unswollen latex particles. Starting with the Stokes expression for centrifugal sedimentation, an equation can be developed for the ratio of the sedimentation coefficient of a particle, S, to the sedimentation coefficient, S, of the same particle having a surface layer ... 

This ratio of the linear velocity of a particle to its angular acceleration, in a centrifugal field, is the sedimentation coefficient (s). As evident from the equation, the term Mb results in the dependence of 5 on the mass, whereas the term/results in its dependence on the size and shape of the macromolecule. 

In another type of experiment with the ultracentrifuge, the sedimentation coefficient of the particle is obtained. If the centrifugal force greatly exceeds the force due to the osmotic field, the particles will sediment, forming a boundary that will move down from the surface of the solution for positive values of (dp/dc2)v.. This movement will be opposed by the buoyancy of the particles and by the frictional force generated by their motion. It can be shown that this leads to a constant velocity (dr/dt) of the boundary and a sedimentation coefficient, s, is defined for the particle by s = (drldt)l(n2r, where r is the distance to the center of rotation (Schachman, 1959). For vanishing particle concentration,... 

What is the velocity of the molecule described in the preceding problem when in a centrifugal field of 75,000 x gl Assume its density to be 1.40 g/cm3. What is its sedimentation coefficient ... 

According to Svedberg and Pederson (1940) the sedimentation coefficient is defined as the sedimentation velocity in a unit field of force (e.g. in a centrifuge) ... 

In the definition, v is the velocity of sedimentation and a is the acceleration of free fall or centrifugation. The symbol for a limiting sedimentation coefficient is [s], for a reduced sedimentation coefficient 5°, and for a reduced limiting sedimentation coefficient [s°] see [l.e] for further details. 

Centrifugation can also be used to study macromolecule-ligand interactions, provided that ligand binding alters the sedimentation coefficient of the macromolecule. The analytical ultracentrifuge is well suited for quantitative studies of binding it is a... 

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