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Isoelectric point

 
Sci-Tech Dictionary: isoelectric point
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(¦ī·sō·i′lek·trik ′pöint)

(physical chemistry) The pH value of the dispersion medium of a colloidal suspension at which the colloidal particles do not move in an electric field.

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Sci-Tech Encyclopedia: Isoelectric point
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The pH of a dispersion medium of a colloidal suspension or an ampholyte at which the solute does not move in an electrophoretic field. The term isoelectric point is abbreviated pI.

Ampholytes are molecules with acid as well as basic functional groups. When dissolved in a suitable medium, ampholytes may acquire positive and negative charges by dissociation or by accepting or losing protons, thereby becoming bipolar ions (zwitterions). Ampholytes may be as small as glycine and carry just one chargeable group each; or as large as polyampholytes (polyelectrolytes that carry positive charges, negative charges, or both). They may possess molecular weights in the hundreds of thousands like proteins or in the millions like nucleic acids, and carry many hundreds of chargeable groups. See also Ion; Nucleic acid; pH; Protein.

An example of establishing the isoelectric point is shown by the course of the pH changes during the titration of alanine [NH2CH(CH3)COOH], a 1:1 ampholyte, meaning a molecule that carries one positively and one negatively ionizable group. Starting from acid solution (see illustration), relatively small pH changes (with alkali as the titrant) are observed between pH 2 and 3 (acidic), and again between pH 9.5 and 10.5 (alkaline), caused by the buffering capacity of the carboxyl (COOH) and amine (NH2) groups as weak electrolytes. The pH for ½ equivalence corresponds to the pK of the acid function (a value related to the equilibrium constant), where one-half of the alanine molecules still carry only a positive charge (−NH3+), while the other half are also negatively charged (−COO−). Thus alanine exists in the form of zwitterions. See also pK.

Titration of alanine with sodium hydroxide (NaOH), showing the course of pH with added fractional equivalents. The four arrows show, from left to right, the p<i>K</i><sub><b>a</b></sub> [½ cations: NH<sub>3</sub><SUP>+</SUP>(CH<sub>3</sub>)COOH, and ½ zwitterions: NH<sub>3</sub><SUP>+</SUP>(CH<sub>3</sub>)CHCOO<SUP>−</SUP>]; the pl (all zwitterions); the p<i>K</i><sub><b>b</b></sub> (½ zwitterions, ½ anions); and the end of titration, when all alanine molecules are in the anionic form: NH<sub>2</sub>(CH<sub>3</sub>)CHOO<SUP>−</SUP>.
Titration of alanine with sodium hydroxide (NaOH), showing the course of pH with added fractional equivalents. The four arrows show, from left to right, the pKa [½ cations: NH3+(CH3)COOH, and ½ zwitterions: NH3+(CH3)CHCOO−]; the pl (all zwitterions); the pKb (½ zwitterions, ½ anions); and the end of titration, when all alanine molecules are in the anionic form: NH2(CH3)CHOO−.

For molecules that carry four or more chargeable groups, that is, for polyelectrolytes, the courses of the overall titration curves may no longer reflect the individual dissociation steps clearly, as the dissociation areas usually overlap. The isoelectric point then becomes an isoelectric range, such as for pigskin (parent) gelatin, a protein that exhibits an electrically neutral isoelectric range from pH 7 to pH 9.

Since ampholytes in an electric field migrate according to their pI with a specific velocity to the cathode or anode, the blood proteins, for example, can be separated by the techniques of gel or capillary-zone electrophoresis. See also Electrophoresis; Titration.

The notion that some ampholytes may pass with changing pH through a state of zero charge (zero zeta potential) on their way from the positively to the negatively charged state has become so useful for specifying and handling polyampholytes that it was extended to all kinds of colloids, and to solid surfaces that are chargeable in contact with aqueous solutions. Practically all metal oxides, hydroxides, or hydroxy-oxides become charged by the adsorption of hydrogen ions (H+) or hydroxide ions (OH−), while remaining neutral at a specific pH. Strictly speaking, the isoelectric point of electrophoretically moving entities is given by the pH at which the zeta potential at the shear plane of the moving particles becomes zero. The point of zero charge at the particle (solid or surface) is somewhat different but often is not distinguished from the isoelectric point. It is determined by solubility minima or, for solid surfaces, is found by the rate of slowest adsorption of colloids (for example, latexes) of well-defined charge.

The important separation technique of ion-exchange chromatography is based on the selective adsorption of ampholytes on the resins with which the column is filled, at a given pH. For example, the larger the net positive charge of an ampholyte, the more strongly will it be bound to a negative ion-exchange resin and the slower will it move through the column. By rinsing with solutions of gradually increasing pH, the ampholytes of a mixture can be eluted and made to emerge separately from the column and be collected. Automated amino acid analyzers are built on this principle. See also Amino acids; Colloid; Electrokinetic phenomena; Ion exchange; Ion-selective membranes and electrodes.

Dental Dictionary: isoelectric point
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n

The pH at which a molecule containing two or more ionizable groups is electrically neutral.

Wikipedia: Isoelectric point
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The isoelectric point (pI), sometimes abbreviated to IEP, is the pH at which a particular molecule or surface carries no net electrical charge.

Amphoteric molecules called zwitterions contain both positive and negative charges depending on the functional groups present in the molecule. The net charge on the molecule is affected by pH of their surrounding environment and can become more positively or negatively charged due to the loss or gain of protons (H+). The pI is the pH value at which the molecule carries no electrical charge or the negative and positive charges are equal.

Surfaces naturally charge to form a double layer. In the common case when the surface charge-determining ions are H+/OH-, the net surface charge is affected by the pH of the liquid in which the solid is submerged. Again, the pI is the pH value of the solution at which the surfaces carries no net charge.

The pI value can 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 uses a pH gradient to separate proteins. Isoelectric focusing is also the first step in 2-D gel polyacrylamide gel electrophoresis.

Contents

Calculating pI values

For an amino acid with only one amine and one carboxyl group, the pI can be calculated from the mean of the pKa's of this molecule.

pI = {{pK_1 + pK_2} \over 2}

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).

pI = {{9.06 + 10.54} \over 2} = 9.80

However, a more exact treatment of this requires advanced acid/base knowledge and calculations.

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.

Ceramic materials

The isoelectric points (IEP) of metal oxide ceramics are used extensively in material science in various aqueous processing steps (synthesis, modification, etc.). For these surfaces, present as colloids or larger particles in aqueous solution, the surface is generally assumed to be covered with surface hydroxyl species, M-OH (where M is a metal such as Al, Si, etc.). At pH values above the IEP, the predominate surface species is M-O-, while at pH values below the IEP, M-OH2+ species predominate. Some approximate values of common ceramics are listed below (Haruta[1] and Brunelle[2], except where noted). The exact value can vary widely, depending on material factors such as purity and phase as well as physical parameters such as temperature. In addition, precise measurement of isoelectric points is difficult and requires careful techniques, even with modern methods. Thus, many sources often cite differing values for isoelectric points of these materials.

Examples of isoelectric points

The following list gives the pH25°C of isoelectric point at 25 °C for selected materials in water:

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.[8] 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[6]). Lewis[6] also lists the IEP of barium titanate, BaTiO3 as being between pH 5 and 6, while Vamvakaki et al.[9] 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.

The farther the pH of an Amino Acid solution is from its pl the greater the electric charge on that population of molecules.

Isoelectric point versus point of zero charge

The terms isoelectric point (IEP) and point of zero charge (PZC) are often used interchangeably, although under certain circumstances, it may be productive to make the distinction.

In systems in which H+/OH- are the interface potential-determining ions, the point of zero charge is given in 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[10]. 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 in a 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[4], 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 neutral net surface charge. The difference between the two, therefore, is the quantity of charged sites at the point of net zero charge. Jolivet uses the intrinsic surface equilibrium constants, pK- and pK+ to define the two conditions in terms of the relative number of charged sites:

pK^- - pK^+ = \Delta pK = \log {\frac{\left[MOH\right]^2}{\left[MOH{_2^+}\right]\left[MO^-\right]}}

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.

References

  1. ^ Haruta M (2004). 'Nanoparticulate Gold Catalysts for Low-Temperature CO Oxidation', Journal of New Materials for Electrochemical Systems, vol. 7, pp 163-172.
  2. ^ Brunelle JP (1978). 'Preparation of Catalysts by Metallic Complex Adsorption on Mineral Oxides'. Pure and Applied Chemistry vol. 50, pp. 1211-1229.
  3. ^ a b c d e f g h i j k l m n o p q Marek Kosmulski, "Chemical Properties of Material Surfaces", Marcel Dekker, 2001.
  4. ^ a b c d Jolivet J.P., Metal Oxide Chemistry and Synthesis. From Solution to Solid State, John Wiley & Sons Ltd. 2000,ISBN 0-471-97056-5 (English translation of the original French text, De la Solution à l'Oxyde, InterEditions et CNRS Editions, Paris, 1994).
  5. ^ U.S. Patent 5,165,996
  6. ^ a b c d e f Lewis, JA (2000). 'Colloidal Processing of Ceramics', Journal of the American Ceramic Society vol. 83, no. 10, pp.2341-2359.
  7. ^ Kosmulski M and Saneluta C (2004). 'Point of zero charge/isoelectric point of exotic oxides: Tl2O3', Journal of Colloid and Interface Science vol. 280, no. 2, pp. 544-545.
  8. ^ Jara, A.A., S. Goldberg and M.L. Mora (2005). 'Studies of the surface charge of amorphous aluminosilicates using surface complexation models', Journal of Colloid and Interface Science, vol. 292, no. 1, pp. 160-170.
  9. ^ Vamvakaki, M., N.C. Billingham, S.P. Armes, J.F. Watts and S.J. Greaves (2001). 'Controlled structure copolymers for the dispersion of high-performance ceramics in aqueous media', Journal of Materials Chemistry, vol. 11, pp. 2437-2444.
  10. ^ A.W. Adamson, A.P. Gast, "Physical Chemistry of Surfaces", John Wiley and Sons, 1997.

Further reading

  • Nelson DL, Cox MM (2004). Lehninger Principles of Biochemistry. W. H. Freeman; 4th edition (Hardcover). ISBN 0-7167-4339-

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