The pI value can also affect the solubility of a molecule at a given pH. Such molecules have minimum solubility in water or salt solutions at the pH which corresponds to their pI and often precipitate out of solution. Biological amphoteric molecules such as proteins contain both acidic and basic functional groups. Amino acids which make up proteins may be positive, negative, neutral or polar in nature, and together give a protein its overall charge. At a pH below their pI, proteins carry a net positive charge; above their pI they carry a net negative charge. Proteins can thus be separated according to their isoelectric point (overall charge) on a polyacrylamide gel using a technique called isoelectric focusing, which utilizes a pH gradient to separate proteins. Isoelectric focusing is also the first step in 2-D gel polyacrylamide gel electrophoresis.
For amino acids with more than two ionizable groups, such as lysine, the same formula is used, but this time the two pKa's used are those of the two groups that lose and gain a charge from the neutral form of the amino acid. Lysine has a single carboxylic pKa and two amine pKa values (one of which is on the R-group), so fully protonated lysine has a +2 net charge. To get a neutral charge, we must deprotonate the lysine twice , and therefore use the R-group and amine pKa values (found at List of standard amino acids).
The pH of an electrophoretic gel is determined by the buffer used for that gel. If the pH of the buffer is above the pI of the protein being run, the protein will migrate to the positive pole (negative charge is attracted to a positive pole). If the pH of the buffer is below the pI of the protein being run, the protein will migrate to the negative pole of the gel (positive charge is attracted to the negative pole). If the protein is run with a buffer pH that is equal to the pI, it will not migrate at all. This is also true for individual amino acids.
Note: The list is ordered by increasing pH values.
Mixed oxides may exhibit isoelectric point values that are intermediate to those of the corresponding pure oxides. For example, Jara et al. measured an IEP of 4.5 for a synthetically-prepared amorphous aluminosilicate (Al2O3-SiO2). The researchers noted that the electrokinetic behavior of the surface was dominated by surface Si-OH species, thus explaining the relatively low IEP value. Significantly higher IEP values (pH 6 to 8) have been reported for 3Al2O3-2SiO2 by others (see Lewis). Lewis also lists the IEP of barium titanate, BaTiO3 as being between pH 5 and 6, while Vamvakaki et al. reported a value of 3, although these authors note that a wide range of values have been reported, a result of either residual barium carbonate on the surface or TiO2-rich surfaces.
In systems in which H+/OH- are the interface potential-determining ions, the point of zero charge is given in the terms of pH. The pH at which the surface exhibits a neutral net electrical charge is the point of zero charge at the surface. Electrokinetic phenomena generally measure zeta potential, and a zero zeta potential is interpreted as the point of zero net charge at the shear plane. This is termed the isoelectric point. Thus, the isoelectric point is the value of pH at which the colloidal particle remains stationary in an electrical field. The isoelectric point is expected to be somewhat different than the point of zero charge at the particle surface, but this difference is often ignored in practice for so-called pristine surfaces, i.e., surfaces with no specifically adsorbed positive or negative charges. In this context, specific adsorption is understood as adsorption occurring the Stern layer or chemisorption. Thus, point of zero charge at the surface is taken as equal to isoelectric point in the absence of specific adsorption on that surface.
According to Jolivet, in the absence of positive or negative charges, the surface is best described by the point of zero charge. If positive and negative charges are both present in equal amounts, then this is the isoelectric point. Thus, the PZC refers to the absence of any type of surface charge, while the IEP refers to a state of net neutral surface charge. The difference between the two, therefore, is quantity of charged sites at the point of net zero charge. Jolivet uses the intrinsic surface equilbrium constants, pK- and pK+ to define the two conditions in terms of the relative number of charged sites:
For large ΔpK (>4 according to Jolivet), the predominate species is MOH while there are relatively few charged species - so the PZC is relevant. For small values of ΔpK, there are many charged species in approximately equal numbers, so one speaks of the IEP.
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