Isoelectric point, definition

Assuming the sensitivity of the determination of oc at the isoelectric point as 1 per cent, the isoelectric point will appear as a real point , if (ar)7> 10-2, that is KJK2> 10-4, whereas for (ar)7< 10-2, that is KJK2< 10-4, a definite interval of pH obeys the isoelectric condition (see Fig. 1.14). 

The precipitate on analysis gave the ratio Ba 90AsaS8. The constancy of the ratio however depends entirely on the size and charge of the arsenious sulphide particles, since the reaction proceeds to the isoelectric point, i.e. electrical neutrality, and is therefore independent of any definite stoichiometric ratio of reactants. 

The net charge on a protein is the algebraic sum of all its positive and negative charges. There is a specific pH for every protein at which the net charge it carries is zero. This isoelectric pH value, termed the isoelectric point, or pi, is a characteristic physicochemical property of every protein. The definition of pi for molecules as complex as proteins is more or less an operational one and is taken to be that pH at which a protein has zero electrophoretic mobility in an isoelectric focusing run. Nevertheless, it has been shown that the pis of some acidic proteins (up to about pH 7) can be calculated from their amino acid compositions.3 5... 

Isoelectric point The pH at which an amino acid has no tendency to migrate under the influence of an electric field. lUPAC convention A set of definitions relating to electrochemical cells and their potentials also known as the Stockholm convention. 

Two parameters were introduced into the description of double electrical layer. One of them is the point of zero charge (PZC) which according to lUPAC definition [101] can be expressed as concentration of potential-determining ions PDI at which the surface charge is equal to zero ( o = 0), as well as the surface potential (V>o = 0). Another parameter is isoelectric point (lEP) defined [101] as concentration of PDI at which the electrokinetic potential is equal to zero (( = 0). 

However, the definition for standardization that Johnston, et al used, referred to the matching of selected polypeptide spot positions as a result of two-dimensional electrophoresis of complex mixtures of proteins and did not refer to standardization in terms of position in the gels i.e., known molecular weight or isoelectric point. At this time, most standards approaches for the commonly used molecular weight ranges are still being directed towards obtaining stable, pure, and well-characterized proteins. 

Here the isoelectric point has been taken to be the pH at which the substance has no net charge. The experimental definition is the pH at which the substance has zero mobility in an electrophoresis cell. In fact, these two values of pH differ so slightly, they are frequently used interchangeably, as is done here.)... 

Solubility. Solubility is an important functional property per se, i.e. in fluid products, and is essential for other functionalities since insoluble proteins can not perform useful functions in foods. The caseins are, by definition, insoluble at their isoelectric points, i.e. in the pH range c. 3.5-5.S the insolubility range becomes wider with increasing temperature. Insolubility in the region of the isoelectric point is clearly advantageous in the production of acid casein and is exploited in the production of two major families of dairy products, i.e. fermented milks and fresh cheeses. However, such insolubility precludes the use of casein in acid liquid foods, e.g. protein-enriched fruit juices or carbonated beverages. Acid-soluble casein can be prepared by limited proteolysis or by interaction with certain forms of pectin. 

See, e.g., D. H. Everett, Manual of Symbols and Terminology for Physi-cochemical Quantities and Units. Appendix II Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Butterworths, London, 1972. When the PZC is measured by an electrokinetic experiment (Sec. 3.4), it is often termed tm isoelectric point (lEP). However, other definitions of the lEP are used in the soil chemistry literature. ... 

Another important point, and one of fundamental significance, is that above the isoelectric point, although no precipitation is observed with added SDS, definite indications of interaction can still be obtained see, for example, the surface tension data for gelatin/ SDS presented below, calorimetric data for BSA/SDS (128), and other information to be given later. This means that proteins, like many HM-polyions, are able to interact with ionic surfactants against an unfavorable (overall) electrical gradient. For this reason many studies involving anionic surfactants have been executed above the i.e.p. of the protein, as illustrated below. 

The usual terms will be used in this discussion, but, for the sake of clarity, it might be well to review briefly the general properties of all enzymes. Enzymes are catalysts and, as such, they lower the free energy of activation of reactive molecules and thus shorten the time required for a reaction to reach equilibrium. Consequently, an enzyme, like a catalyst, does not favor any equilibrium position but rather it accelerates equally the reactions in both directions in a system approaching equilibrium. All enzymes thus far characterized show the properties of proteins, and all enzymes that have been crystallized are definitely proteins. Enzymes, therefore, are subject to denaturation by heat and other physical and chemical agents, have isoelectric points, and are synthesized only by a living cell. 

Amphoteric surfactants by definition are chimeric, exhibiting anionic character in alkaline solution, nonionic character near their isoelectric point, and cationic character in acidic solution [73]. As a result of their complex charge characteristics, their interactions with interfaces must be examined iudividually and as a function of pH. For example, the adsorption of alkyl betaines firom solution onto wool keratin is much greater at acidic than alkalide pH values. Although amphoteric surfactants are used extensively to improve the cosmetic attributes of many consmner products [74], their interactions with skin have received little attention. 

It should be noted that the equilibrium conditions, Eqs. (2) and (6b), have the same form whether there is adsorbed surface charge or not. The magnitudes of (0) and Vc(0), however, will differ in these two cases because of the difference in the charge neutrality conditions, Eqs. (4) and (6a). An important consequence of the above consideration is that in the presence of adsorbed surface charge and constant x potential the space charge potentials Vs(0) and Fc(0) will not be zero at Cf/O), and in fact will not be zero at the same value of silver ion concentration in solution. The definition of isoelectric point therefore becomes ambiguous and it is necessary to define isoelectric point separately for each side of the interface corresponding to values of silver ion concentration in solution where F (0) = 0 or Fc(0) = 0. If only K (0) is measured, as in the electrophoresis experiment, the isoelectric point determined for F (0) = 0 does not ensure that Kc(0) is zero at the same time. 

The distinction between the isoelectric and isoionic states of a protein was first made in a classic paper by S0rensen et at. (1926). Three definitions of the isoionic point were proposed, one of these being the stoichiometrically defined point which we have called the point of zero net proton charge. The other tw o were operational definitions (summarized by Linderstr0m-Lang and Nielsen, 1959). The term isoionic point, as used here, corresponds to one of these two operational definitions, chosen because it always permits calculation of the point of zero net proton charge, which is the only parameter of real interest in the analysis of titration curves. The same choice has been made by Scatchard and Black (1949). 

The isoionic point, for a protein solution not too low in concentration, is experimentally defined as that pH which does not change when a small amount of pure protein is added to the solution (46). This definition also applies to a protein which has been focused isoelectrically (7,26). If more protein is added to the zone, the pH does not change. The buffering at the site of focusing is considered to be completely dominated by the... 

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