Electrophoresis

Electrophoresis ( cataphoresis outdated ) refers to the migration of colloidal particles by an electric field.

Description

The drift velocity (also called " migration rate " ) v of the colloidal particles, typically proteins or nucleic acids, wherein the electrophoresis is proportional to the field strength E and the ion charge Q is inversely proportional to the particle radius r, and the viscosity η of the material. Also plays the ion milieu of the solution flowing in the electric current, a significant role. Physical cause of this movement is the shear force in the electric double layer, which is surrounding the colloid (star double layer), and the charged fluid in relative movement to the macromolecule. In gel electrophoresis, makes the relationship between the particle radius and the pore size of serving as a carrier medium gel a role, because the gel acts as a molecular sieve, so that a larger particle radius more inhibitory effect on the migration rate, as can be expected only by the viscosity alone would. Due to the different ionic charge and the particle radius, the individual substances (molecules) move at different speeds through the carrier material and achieve a separation according to their electrophoretic mobility. Thus, the electrophoresis is very well suited for the separation of mixtures of substances (in particular molecule mixtures). As support material, liquids, gels (see gel electrophoresis, usually with polyacrylamide or agarose) come or solids used.

Agarose gels are especially in the separation of DNA fragments for use as most proteins are separated in polyacrylamide gels. As methods for proteins are SDS -PAGE and Western blot are used. Proteins must be present as zwitterions with additional charges by a detergent such as sodium dodecyl sulfate (English sodium dodecyl sulfate, SDS) are loaded to come from a separation according to the heterogeneous charge densities to a separation according to molecular mass. By the addition of SDS and boiling ( denature ) the proteins adsorb proportional to its unfolded length (and also proportional to the molecular weight ) of the aliphatic end negatively charged sodium lauryl sulfate. This bind approximately 1.4 grams of SDS per gram of protein in SDS -percent solutions. The negatively charged sulfate groups of SDS molecules repel each other, which the unfolding (linearization ) promotes the proteins, where the protein has no disulfide bridges. Therefore, in addition added at the molar mass reducing agent for the conversion of disulfides to thiols. Since several hundred negatively charged SDS molecules bind to the protein molecules, the self- charge of the proteins in the basic pH of the gel can be neglected.

Electrophoretic mobility

The electrophoretic mobility of two particles to be separated must be different in order to achieve a separation by electrophoresis. The electrophoretic mobility is the sum of many physical factors that ultimately influence the migration velocity of a particle during electrophoresis. Generally the driving force that causes the movement of the particles, the force F acting on a particle with a certain charge q within an electric field with given field strength E.

This is counteracted by a first force applied by the viscosity and the size of the particle ( for idealized spherical particles: ) results, and can be calculated according to Stokes' law.

From these two equations gives the theoretical electrophoretic mobility. Theoretically, for the reason, as these two equations (practically salt-free, but this contradicts the principle of electrophoresis because salt ions are required as mobile charge carriers ) only for an idealized, carrier-free state with infinitely dilute electrolyte apply. It is also assumed here that the accelerating force of the friction force equal and therefore prevails a constant migration velocity. Therefore, the ion mobility results in this model as follows:

In real systems, other factors such as the friction between the hydrate get ( electrophoretic effect), the deformation of the charge distribution as a relaxation of the electric field ( dissipative effect, see Ion atmosphere), the degree of dissociation of the electrolyte and effects of the carrier material ( molecular sieve, electroosmosis, and adsorption ) for supporting.

While traditional theories assume that electrophoretic activity of a particle implies a net charge of the particle, create new results suggest from molecular dynamics simulations, that due to the molecular structure of water can show electrophoretic activity at the surface and uncharged particles.

Species

• Discontinuous electrophoresis
• Gel electrophoresis
• Capillary electrophoresis
• Gradient electrophoresis
• Pulsed -field gel electrophoresis
• Dichtegradientenelektrophorese ( carrier-free electrophoresis)
• Electrofocusing
• Lipid
• Serum electrophoresis
• Two-dimensional electrophoresis
• Free- flow electrophoresis
• Electro-osmosis occurs in electrophoretic processes
• Isoelectric focusing
• Überwanderungselektrophorese
• SDS-PAGE ( SDS-polyacrylamide gel electrophoresis)
• Capillary electrochromatography
• Affinity electrophoresis
• Zone electrophoresis
• Immunoelectrophoresis

Application

Is applied electrophoresis primarily as a method of analysis in biology and medicine. To the main applications include DNA analysis in the form of fragments and DNA sequencing. Here, the possibility is used to separate molecules of different lengths from each other. To determine the values ​​of a gel such as run lengths, molecular weights, quantifications or normalizing a specialized analysis software is utilized. Also the separation of proteins and for the highly technological method of proteomics is the electrophoresis, the foundation. The graphical representation of the results is an electropherogram. In addition to the analytical and preparative electrophoretic methods are used for production of milligram quantities of purified proteins are used ( inter alia, free-flow electrophoresis).

Further, technical applications:

• Electronic Paper
• Cathodic dip painting
• Electrofiltration

History

Electrophoresis was developed in 1937 by Arne Tiselius, could be separated as a method with the colloids in a carrier liquid in an electric field (english moving boundary electrophoresis, electrophoresis with a movable boundary layer '). Tiselius was awarded the 1948 Nobel Prize in Chemistry. In the forties of the 20th century and the growing solid phases for better separation were used (English zone electrophoresis, zone electrophoresis ' ) as the starch gel by Oliver Smithies or filter paper. Since these tend to microbial decomposition, other hydrogels were used in sequence, such as agarose or polyacrylamide. While in the fifties often still a radial electrophoresis was carried out on round disks (English radially electrophoresis, disc electrophoresis '), almost exclusively rectangular gels are used (English slab gel electrophoresis, electrophoresis gel plates ').