PH value of the isoelectric point

The pH at which the molecule carries no net charge is called the isoelectric point (pl) for alanine the isoelectric point is pH 6.0. The molecule still carries charges, but they neutralize each other. The pH value of the isoelectric point is simply the average of the two pA" values ... 

The adsorption of HPF (initial Chpf=0.0795 wt% in the solution) is maximal at pH 5-6 which is close to the pH value of the isoelectric point (lEP) of HPF. The adsorption slightly decreases (by 20% at pH 8) at pH >6 (Figure 6.14b) because the adsorption of proteins is maximal at their lEP. Consequently, the state of HPF is stable at pH 5-8 that is of importance because HPF can easily denature in strongly acid or base solutions, as weU as on the adsorption. A similar type of the pH dependence of the adsorption is characteristic for the systems, in which interaction of proteins with a silica surface occurs predominantly due to electrostatic forces and the hydrogen bonds (the main portion of which is due to electrostatic forces). 

To an extent the surface charges are determined by the pH of the solution, and by the isoelectric point of the oxide, i.e. the pH at which the oxide surface is neutral. The surface is negative at pH values below the isoelectric point and positive above it. Obviously, the charged state of the surface enables one to bind catalyst precursors of opposite charge to the ionic sites of the support. 

The aqueous phase pH determines the ionization state of the surface-charged groups on the protein molecule. Solubilization of the protein in RMs is found to be dominated by electrostatic interactions between the charged protein and the inner layer of the surfactant head groups [112]. Solubilization of protein is favored at pH values above the isoelectric point (pi) of the protein in the case of... 

In summary, there are two kinds of charges, namely those which are pH independent and those which are pH dependent. The latter may be either positive or negative, depending on the value of the isoelectric point (3). The former are negative and either localized or delocalized. 

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... 

The separation conditions described in Table V show that a low pH (2.S-3.5) is preferable for almost all protein separations. This pH value is compatible with reversed-phase columns and ensures that the silanol groups present in the packing are not ionized. For most proteins it is best to avoid a pH value near the isoelectric point of the protein to minimize the possibility of precipitation e.g., insulin is insoluble at pH values near its p/ of 5.5. Since all carboxylate anions present in proteins will be protonated at pH values of less than 3, the use of low pH values in reversed-phase HPLC should ensure that the protein samples are cationic and thus minimize any aggregation effects. Some proteins, however, may be unstable at low pH values and therefore higher pH values may be... 

With its determination carried out at constant ionic strength the pH, value, so obtained, provides an assessment of the acidity of the amphoteric ion exchanger. This pH, relatable to the average value of pKj and pK2 with Eq. (16), is very close to the value of the isoelectric point 0 )- Paries [105] has pointed out that the relationship between the lEP of a solid surface and the valency-effective ionic radii, when corrected for crystal field effects, coordination, hydration, and other factors, is quite good. He also has indicated that the broad probable lEP range characteristic of a cation oxidation state may be selected from the data in Table 2 as shown below. It is known that the lEP for amphoteric oxides is affected by the presence of impurities, crystallinity and the chemical species under investigation. 

Thermal gelation of a protein is influenced by many factors, such as the protein concentration, pH, and presence of salts. Protein-Protein interactions are generally favored at conditions that reduce the net charge on the molecules, that is, pH values near the isoelectric point (Boye et al., 1995). Monovalent and divalent salt ions screen electrostatic interactions between charged protein molecules (Bryant... 

The soil pH-mineralogy interactions are shown in Figure 4.3. At pH values above the isoelectric point of 7.1 for ricin, there is very little ricin sorbed within the clay lattice (part a). As the soil pH decreases, more ricin is trapped within the clay interlayer. 

Some reported values of the isoelectric point are based on the change by carbon of the pH of buffered solutions, the buffer solution whose hydrogen ion concentration is unaffected being considered to represent the isoelectric pH of the carbon. Values thus obtained range from pH 5 to 9 and appear to depend on the selective adsorb-ability of the separate constituents of the buffer. Therefore, they bear no relation to isoelectric points determined electrokinetically, and have no real significance for such purpose (see 9 16). 

Cationic polymers are of particular interest carriers in drug and gene deliveiy because of their ability to promote cellular uptake.This holds also for the delivery of proteins, especially for those that possess an overall anionic charge at pH values above the isoelectric point (p/). These proteins can form soluble, nanosized, polyelectrolyte complexes with natural or synthetic cationic polymers by simply mixing the oppositely charged protein and polymer that self-assemble by electrostatic attraction, as represented in Scheme 14.1. 

---