Isoelectric point for

G. A. Parks, Chem. Revs. 65, 177—198 (1965). A comprehensive list of isoelectric points for soHd oxides and hydroxides. 

Surfa.ce (Charge. Inorganic particles have a surface charge in water that is a function of both the particle s character and the pH of the water. Each particle has an isoelectric pH value where the negative and positive charges on the surface just neutralize each other. Isoelectric points for some common inorganic particles are shown in Table 1. 

At the pH = Jt there is a balance of charge and there is no migration in an electric field. This is referred to as the isoelectric point and is determined by the relative dissociation constants of the acidic and basic side groups and does not necessarily correspond to neutrality on the pH scale. The isoelectric point for casein is about pH = 4.6 and at this point colloidal stability is at a minimum. This fact is utilised in the acid coagulation techniques for separating casein from skimmed milk. 

Thomson MOW Click Organic Interactive to learn to estimate isoelectric points for simple amino acids and peptides. 

The isoelectric point for lysine is that pH at which the compound is in an electrically neutral form, and this will be the average of pATa2 (the cation) and pAfa3 (the dipolar ion). For lysine, pATa2 = 8.95 and pATa3 = 10.52, so pi = 9.74. 

I 21.10 Predict whether the isoelectric points for the following a-amino acids are considerably acidic, slightly acidic, or basic (a) alanine, (b) lysine, (c) aspartic acid, (d) cystine, (e) tyrosine. (See Table 21-1 and Problem 21.6.) ... 

Use these data to evaluate the isoelectric point for Th(OH)4. Since H+ and OH- appear to be potential determining, p may be estimated at various pH levels according to Equation (11.1) if we identify the isoelectric point with the true point of zero charge. Compare these values... 

How many components does this sample apparently contain What are the values of the isoelectric points for each ... 

Hydrolysis. The surfaces of metal oxides and hydroxides can take up or release H+ or OH- ions and become chaiged. Potentials as high as 100 mV may be sustained in aqueous solutions. For aqueous solutions this is a function of the pH the zeta potential for the particle is positive if the solution pH is below the particle s isoelectric pH (pH ), and negative if the pH is above pH. Isoelectric points for metal oxides are presented in several publications (22,23). Reactions of hydroxyl groups at a surface, Q, with acid and base may be written as follows ... 

Calculate the isoelectric point for histidine, aspartic acid, and arginine. Calculate the fractional charge for... 

The surface of uncalcined aluminas consists of amphoteric hydroxyl groups that are even less acidic than those in silica gel. Electrophoretic measurements by Stigter et al. (90) show that the isoelectric point for alumina is attained at a pH of 9, much higher-than the value of 2 obtained in the case of silica gel. Thus, hydroxylated aluminas are relatively non-acidic, unless they are promoted with acid-producing impurities. 

Table 1. Isoelectric Points for Anti-Lactose Isoforms...

The pH at which the molecule carries no net charge is called the isoelectric point. For glycine the isoelectric point is pH 6.0. Of course, in a solution of glycine at pH 6, at any instant there will be some molecules that exist as COOH—CH2— NH3, and an equal number as COO-—CH2—NH2, and even fewer of COOH—CH2—NH2. Because of the symmetry of the titration curve around the isoelectric point, it is possible to calculate the pH of the isoelectric point, given the individual pKa values. 

For most oxides, as the pH is increased, the adsorption of potential determining ions, H" and OH, changes in correspondence with the concentration of these species in solution. For each surface, therefore, a point is reached at which the concentration of positive ions and negative ions just balance, the point of zero charge. The pH where the zeta potential, is 0, is called the isoelectric point. The isoelectric point for various ceramic materials is given in Table 9.11. 

The properties of the isolated peptides were quite similar in nature, whereby each peptide consisted of 12 amino acids in length and possessed a munber of residues with functional side groups that could stabilize nanoclusters. In many instances, these side chains were the hydroxyl-terminated side chains of serine, threonine, and tyrosine. In two of the peptides (AG3 and AG4), the location of the hydroxylated amino acids was conserved within two of the peptides. Similarly, one proline amino acid was conserved throughout all three of the sequences. Upon incubating each peptide in a solution of silver nitrate with no exogenous reductant, a clearly observable plasmon resonance peak arose at 440 nm for AG3 and AG4, but not with AG5. The peak was quite broad, indicative of a disperse size and shape distribution. The main difference between the active peptides and inactive AG5 was an overall basic isoelectric point for AGS The assays were performed at neutral conditions which would modulate the side-chain dynamics under acidic or basic conditions. 

Abrasive particles are a key component in CMP slurry. The most commonly used abrasive particles include silica, alumina, ceria, zirconia, titania, and diamond. Table 21.1 listed a set of information on each type of abrasive particles such as density, microhardness, and isoelectric points (lEP). It is important to point out that the specific values for these properties depend highly on the preparation techniques and the specific states of the samples. The values listed in the table represent an average of the most commonly reported data. For example, the isoelectric point for silica is a function of the number of hydroxyl groups, type and level of adsorbed species, metal impurity in the solid matrix, and the treatment history of the materials [1]. There are three major types of silica according to their preparation methods fumed, colloidal, and precipitated. The common sources for obtaining these abrasive particles are listed in Table 21.2. As examples, some of the more specific information on... 

It must be pointed out that in general it is not possible to obtain an isoelectric point for a pure ampholyte, since equation (13) for the hydrogen ion concentration in the latter will be equal to that given by (14) for the isoelectric point in special cases only. It is usual, therefore, to add a small amount of alkali or acid in order to obtain an isoelectric solution. At the isoelectric point an amino-acid dissociates equally to yield RHt and R " ions, but if the hydrogen ion concentration exceeds the theoretical value for this point, there will be a tendency for the hydrogen ions to react with R- or RH= = to yield RH= = or RHj, respectively. In a solution of smaller hydrogen ion concentration, however, the RHt or RH" = ions will tend to ionize further. These conclusions have been utilized to determine the isoelectric points of amino-acids a small amount of ampholyte is added to each of a series of solutions of known pH these solutions should not be too strongly buffered. If there... 

Calculate the isoelectric point for each amino acid. 

Thus alumina membranes surface modified with silanes and sulfone [Shimizu et al. 1987] and with trimethyl chlorosilane TMS [Shimizu et al. 1989] and glass membranes adsorbed with surfactants [Busscher et al. 1987] have been studied this way. The results show that surface treatments alter the zeta potentials. Shimizu et al. [1989] have also demonstrated that under normal operating conditions the zeta potentials of alumina membranes do not change over time even for a period of two to three years. The isoelectric point for alumina particles thus determined is close to 4.00 as determined by direct measurement of membranes. 

The ester formed above is heated under reflux for 9 hours with 50 ml. of 48% aqueous hydrobromic acid. The reaction mixture is dissolved in 250 ml. of hot water and allowed to cool overnight. The precipitated benzoic acid is removed by filtration, and the filtrate is evaporated to dryness under reduced pressure. The solid residue is dissolved in 50 ml. of hot water, and the solution is brought to pH 6 (isoelectric point for leucine) with 6 i T ammonium hydroxide solution, after which 200 ml. of 95% ethanol is added. Cooling in the refrigerator for 36 hours allows precipitation of the amino acid. The yield is 6.3 g., or 78%. The product may be recrystallized from a mixture of water and ethanol. 

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