Ultracentrifugation velocity

Micellar diameter and size polydispersity can be obtained directly in water or in an isotonic buffer by DLS. DLS can also provide some information on the sphericity of polymeric micelles (Kataoka et al., 1996 Nagasaki et al., 1998). Ultracentrifugation velocity studies are sometimes performed to assess the polydispersity of polymeric micelles (Yokoyama et al., 1994 Hagan et al., 1996). 

Apart from tliese mainstream metliods enabling one to gain a comprehensive and detailed stmctural picture of proteins, which may or may not be in tlieir native state, tliere is a wide variety of otlier metliods capable of yielding detailed infonnation on one particular stmctural aspect, or comprehensive but lower resolution infonnation while keeping tlie protein in its native environment. One of tlie earliest of such metliods, which has recently undergone a notable renaissance, is analytical ultracentrifugation [24], which can yield infonnation on molecular mass and hence subunit composition and their association/dissociation equilibria (via sedimentation equilibrium experiments), and on molecular shape (via sedimentation velocity experiments), albeit only at solution concentrations of at least a few tentlis of a gram per litre. 

Since the radial acceleration functions simply as an amplified gravitational acceleration, the particles settle toward the bottom -that is, toward the circumference of the rotor-if the particle density is greater than that of the supporting medium. A distance r from the axis of rotation, the radial acceleration is given by co r, where co is the angular velocity in radians per second. The midpoint of an ultracentrifuge cell is typically about 6.5 cm from the axis of rotation, so at 10,000, 20,000, and 40,000 rpm, respectively, the accelerations are 7.13 X 10, 2.85 X 10 , and 1.14 X 10 m sec" or 7.27 X 10, 2.91 X 10, and 1.16 X 10 times the acceleration of gravity (g s). 

The ultracentrifuge has been used extensively, especially for the study of biopolymers, and can be used in several different experimental modes to yield information about polymeric solutes. Of the possible procedures, we shall consider only sedimentation velocity and sedimentation equilibrium. We shall discuss these in turn, beginning with an examination of the forces which operate on a particle setting under stationary-state conditions. 

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. 

With sedimentation velocity we measure the change in solute distribution across a solution in an ultracentrifuge cell as a function of time. An example of such a change is given in Fig. 2a for potato amylose [29]. 

The quantity on the left, which is determined by measuring the rate of movement of the sedimentation boundary in the ultracentrifuge operated at the constant angular velocity co, is called the sedimentation constant s. Thus... 

A nice example for the utilization of scaling laws in sedimentation velocity runs with the analytical ultracentrifuge has been published by Machtle and Borger [78]. For the polyelectrolyte sodium polystyrene sulfonate (NaPSS) in 0.5 molar NaCl solution they found a scaling law for the sedimentation coefficient at... 

Equations suitable for simulation of molecular weight distributions for any initial distribution and chosen values of G(scission) and G(crosslinking) have been developed and demonstrated. The molecular weight distributions may be obtained by GPC (with the limitation of changes in relative hydrodynamic volumes) and by sedimentation velocity in the ultracentrifuge. 

During ultracentrifugation, when the frictional force equals the centrifugal force and the particle achieves maximum velocity, the relative molecular mass... 

These solutions have been examined in sedimentation velocity runs on the analytical ultracentrifuge (31). Beyond 0.5 base equivalent per mole of iron a fairly narrow sedimentation peak developed. The sedimentation coefficient, 7 1 S, was essentially constant up to 2.5 base equivalents per mole of iron, although the area under the peak increased with increasing degree of hydrolysis. Apparently, then, hydrolysis of ferric nitrate beyond the reversible equilibrium region produces increasing amounts of a fairly discrete high polymer whose size is constant. 

As described before, TLC separation based on the phase-separation mechanism causes fractionation primarily by molecular-weight differences. The resolution attained by this mechanism in TLC will be discussed below by comparing it with those attained by GPC and velocity ultracentrifugation. To this end, some findings obtained by Miyamoto etal. during the course of a study on TLC separation of star-shaped polystyrenes96 97) are referred to. 

Fig. 10. Velocity ultracentrifugation pattern observed for sample S-A dissolved in cyclohexane at 0.077 g/dl. For further operation conditions, see text...

Most probable settling velocity from sedimentation data Particle-size determination from sedimentation equation Sedimentation in an ultracentrifuge Solvation and ellipticity from sedimentation data Diffusion and Gaussian distribution Temperature-dependence of diffusion coefficients... 

Sedimentation velocity measurements were made on a Beckman-Spinco Model E ultracentrifuge equipped with RTIC. The sedimentation coefficients were not measured as a function of concentration for... 

---