Svedberg

The sedimentation coefficient is the ratio of the maximum sedimentation velocity of a particle in a centrifuge divided by the strength of the centrifugal field. The dimension of the coefficient is time his unit is Svedberg, abbreviated S, corresponding to 10-13 s is the unit named after the Swedish chemist Theodor Svedberg.

Properties

The size of the sedimentation coefficient depends on the mass and shape of the particle, as well as its interaction with the medium in which the sedimented particles. It can therefore, when using a medium of known characteristics to determine the nature of the particle, and in particular of its mass, may be used. The masses of very small particles are mainly in biology as determined by a centrifuge, for example ribosomes, virions, protein molecules. In order to obtain a sufficient centrifugal field, with such small particles, ultracentrifuges are used usually.

During centrifugation results in a centrifugal field, whose strength is at an angular velocity ω ( = 2π · rotation frequency ) at a distance r from the axis of rotation ω2 · r. On a particle acts in this distance a centrifugal force equal to me the product of the effective mass of the particle and the strength of the centrifugal field is therefore equal to me · ω2 · r. Here is the effective mass me decisively which is lower due to the buoyancy of the particle in the medium than the actual mass m. Between the actual mass m and the effective mass m, the relation me = m (1 - V · ρ ), where V is the partial specific volume of the particle and ρ is the density of the medium.

The centrifugal force accelerates the particles radially outwards. With increasing velocity of the particle, the friction between the particles sedimenting and the surrounding medium is increased. The frictional resistance f · V is proportional to the sedimentation velocity V, the factor of proportionality f is the coefficient of friction of the particle in this medium, which depends on the shape, size, and the hydration of the particle and on the viscosity of the medium. Sedimentation of the particle is accelerated until the friction resistance is equal to the centrifugal force, so as to compensate is to so f = v · Me · ω2 · r. From then on, the particles sedimented at a constant speed v. Dividing this sedimentation velocity v by the strength of the centrifugal field ω2 · r, we obtain a quantity that depends only on the nature of the particle and its coefficient of friction in the medium in question, with can be thus determined with a known interaction with the surrounding medium, the condition (among mass ) of the particle. This size is called the sedimentation coefficient s.

Its dimension is given by

In determining the mass of particles from its sedimentation coefficient is taken into account that two particles of the same mass can have different sedimentation coefficients, if they have different densities ( influence on the lift ) and forms (influence on the friction coefficient). Therefore, the sedimentation coefficients can not simply be added even when common storage of two particles. For example, a complex ribosome consists of two ribosomal subunits of 30 S and 50 S has no sedimentation coefficient of 80 S, but "only" 70 pp.

Is the sedimentation coefficient of two substances each of the concentration of the other substance dependent, it is observed that the more slowly sedimenting material is relatively concentrated for faster higher than expected. This is called Johnston - Ogston effect, which is named after the biochemists Joseph Johnston and Alexander Ogston.

Determination of the sedimentation coefficient

The sedimentation coefficient of a particle can be determined by centrifugation. The traveling time is sigmoidal concentration gradient along the length of the gradient, the inflection point corresponds approximately to the position of the boundary layer. With the migration distance x from the rotor center at a time t and ω can the angular velocity of the sedimentation coefficient can be determined.