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The sedimentation coefficient is typically dependent on the concentration of the solute (i.e. a macromolecular solute such as a protein). Despite 80+ years of study, there is not yet a consensus on the way to perfectly model this relationship while also taking into account all possible non-ideal terms to account for the diverse possible sizes, shapes, and densities of molecular solutes. [2]
A laboratory ultracentrifuge. In chemistry, a Svedberg unit or svedberg (symbol S, sometimes Sv [a]) is a non-SI metric unit for sedimentation coefficients.The Svedberg unit offers a measure of a particle's size indirectly based on its sedimentation rate under acceleration (i.e. how fast a particle of given size and shape settles out of suspension). [1]
It can be used to estimate the time (in hours) required for sedimentation of a fraction with a known sedimentation coefficient (in svedbergs): t = k s {\displaystyle t={\frac {k}{s}}} The value of the clearing factor depends on the maximum angular velocity ω {\displaystyle \omega } of a centrifuge (in rad/s) and the minimum and maximum radius ...
The simplest measurement to be obtained is the sedimentation coefficient, which depends upon the size of the molecules being sedimented. This is the ratio of a particle's sedimentation velocity to the applied acceleration causing the sedimentation.
The theoretical basis for the measurements is developed from the Mason-Weaver equation. The advantage of using analytical sedimentation equilibrium analysis for Molecular Weight of proteins and their interacting mixtures is the avoidance of need for derivation of a frictional coefficient , otherwise required for interpretation of dynamic ...
where t is the time, c is the solute concentration (moles per unit length in the z-direction), and the parameters D, s, and g represent the solute diffusion constant, sedimentation coefficient and the (presumed constant) acceleration of gravity, respectively. The Mason–Weaver equation is complemented by the boundary conditions
Sedimentation depends on mass, shape, and partial specific volume of a macromolecule, as well as solvent density, rotor size and rate of rotation. The sedimentation velocity can be monitored during the experiment to calculate molecular weight. Values of sedimentation coefficient (S) can be calculated. Large values of S (faster sedimentation ...
The first and second terms on the right-hand side of the Lamm equation are proportional to D and sω 2, respectively, and describe the competing processes of diffusion and sedimentation. Whereas sedimentation seeks to concentrate the solute near the outer radius of the cell, diffusion seeks to equalize the solute concentration throughout the cell.