Electrophoretic mobility medium

The velocity with which a solute moves through the conductive medium due to its electrophoretic mobility (Vep). 

The reaction center is built up from four polypeptide chains, three of which are called L, M, and H because they were thought to have light, medium, and heavy molecular masses as deduced from their electrophoretic mobility on SDS-PAGE. Subsequent amino acid sequence determinations showed, however, that the H chain is in fact the smallest with 258 amino acids, followed by the L chain with 273 amino acids. The M chain is the largest polypeptide with 323 amino acids. This discrepancy between apparent relative masses and real molecular weights illustrates the uncertainty in deducing molecular masses of membrane-bound proteins from their mobility in electrophoretic gels. 

If the electric field E is applied to a system of colloidal particles in a closed cuvette where no streaming of the liquid can occur, the particles will move with velocity v. This phenomenon is termed electrophoresis. The force acting on a spherical colloidal particle with radius r in the electric field E is 4jrerE02 (for simplicity, the potential in the diffuse electric layer is identified with the electrokinetic potential). The resistance of the medium is given by the Stokes equation (2.6.2) and equals 6jtr]r. At a steady state of motion these two forces are equal and, to a first approximation, the electrophoretic mobility v/E is... 

In order to influence a migration it is obvious that one can alter the charge of the compounds, the viscosity of the medium and the dynamic radius of the compounds. According to Eq. 17.5, the electrophoretic mobility is the proportionality factor in the linear relationship of the migration velocity and the electric field strength... 

Fig. 10.9 Electrophoretic mobility of synthetic Fe oxides (0.01 g L" ) in the presence of humic material from a eutrophic lake. Curve a goethite in an Na -Cl"-HCOi medium, total ionic strength 2-10" M, no humic material. Curve b ...

The constant of proportionality in Equation 6.8 is called the electrophoretic mobility and expresses the velocity of the ion (in m s ) in the considered medium per unit electric held (in V m ) ... 

Therefore, the electrophoretic mobility is expressed in m V s, and is a characteristic constant for any given couple ion-medium. ... 

Note e = electron charge NA = Avogadro s number z, = charge of ion of type Mt = molar concentration of ions in the bulk e = dielectric constant of the medium 4 = energy of attraction A = Hamaker constant d = distance between the surfaces 4 = energy of repulsion = ionic concentration (in number/volume) T0 1 17 = viscosity of the liquid u = electrophoretic mobility... 

Another important and effective use of electrophoresis for the analysis of proteins is isoelectric focusing (IEF), which examines electrophoretic mobility as a function of pH. The net charge on a protein is pH dependent. Proteins below their isoelectric pH (pHj, or the pH at which they have zero net charge) are positively charged and migrate in a medium of fixed pH toward the negatively charged cathode. At a pH above its isoelectric point, a... 

Studies on electrophoretic mobility have provided additional data on the excess of charge at the interface between suspended matter and electrolytic medium. Particles in suspension in fresh, sea and estuarie waters appear ubiquitously to exhibit a small range of negative surface charge. This uniformity is attributed to the presence of organic surface coatings on the particles. 

Model particle mobility has been determinated with the Tiselius method (Tiselius, 1937, 1938). This method also allows the integration of the mobility of a large number of particles even if the refractive index is very close to that of the electrolyte medium, allowing to minimize the experimental errors inherent to the classical microelectrophoretic techniques. The electrophoretic mobilities will not be transformed into surface charges because the theoretical relationship between these parameters is highly dependant on the particle radius of curvature and the electrolyte concentration in the vicinity of the particle (Hunter and Wright, 1971). For both methods, the analytical error falls below 5 %, however, it increases up to 10 % for natural composite samples and/or low mobilities. 

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