Differential migration value

Proteins contain a variety of functional groups and interact with the stationary phase at a number of simultaneous sites on the protein molecule, each more or less affected by change in eluent, or mobile-phase, pH or ionic strength. The equilibrium constant for the dissociation of the adsorption complex thus contains a product of many eluent-sensitive concentration terms, and the equilibrium position is very sensitive to elution conditions. Under the elution conditions, some proteins in a mixture may be tightly bound by the stationary phase (t), oo) while others are unretained (t R 0). Differential migration (Section 19.2.1) is replaced by extreme retention values. 

This constant is a true thermodynamic value which is temperature dependent it expresses the relative tendency of a solute to distribute itself between the two phases. Differences in distribution constants result in differential migration rates of solutes through a column. 

The acid-base properties, and hence ionic character, of peptides and proteins also can be used to achieve separations. Ion-exchange chromatography, similar to that described for amino acids (Section 25-4C), is an important separation method. Another method based on acid-base character and molecular size depends on differential rates of migration of the ionized forms of a protein in an electric field (electrophoresis). Proteins, like amino acids, have isoelectric points, which are the pH values at which the molecules have no net charge. At all other pH values there will be some degree of net ionic charge. Because different proteins have different ionic properties, they frequently can be separated by electrophoresis in buffered solutions. Another method, which is used for the separation and purification of enzymes, is affinity chromatography, which was described briefly in Section 9-2B. 

While a knowledge of surface mobility is of great interest in physical adsorption, it becomes essential in chemisorption phenomena. For instance in calorimetric work a curve of differential heats of adsorption versus surface coverage will be horizontal if adsorption is localized but shows the customary slope from high to low values of the heat of adsorption if the adsorbed layer is mobile Furthermore if a chemisorbed intermediate takes part in a surface reaction (crystal growth, corrosion, catalysis), it is essential to know whether, after adsorption anywhere on the surface, it can migrate to a locus of reaction (dislocation, etch pit, active center). Yet here again, while Innumerable adsorption data have been scrutinized for their heat values, very few calculations have been made of the entropies of chemisorbed layers. A few can be found in the review of Kemball (4) and in the book of Trapnell (11). 

TLC is a separation technique in which the components of a hpid mixture are differentially distributed between a solid stationary phase, spread as a thin layer on a plate made of inert material, and a solvent mobile phase. Depending on their type, the components are retained with different strengths on the layer to give distinctive spots or bands. The migration of a band is presented quantitatively by the corresponding R value. The stronger the retention, the lower the Rf value becomes. 

Phenomenological modeling uses a set of partial differential equations that characterize the fines migration process by means of model parameters. The values of these phenomenological parameters are attained through experiments. Phenomenological modeling can also be... 

An injection of l,25-(OH) D- causes a rapid rise in calcium transport to a peak value at 6 hours, followed by a decline to a low value at 12 hours, and a rise to a second maximum at 24 hours, which is sustained for several days. A second injection of l,25-(OH)2D2 after the 24-hour period results in a super-induction of the initial response in addition to the second response. These findings suggest that there are at least two mechanisms of intestinal calcium transport responsive to 1,25-(0H)2D2. The first response undoubtedly represents the transport activity of existing villus cells, whereas the second response likely results from an effect of 1,25-(OH)2D, on the crypt cells that then differentiate and migrate up the villus region to promote intestinal calcium transport. A similar blphasic response has been observed for... 

The difference in partition coefficient values leads to differential rates of migration of the analytes, thus affecting a separation. 

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