Isoelectric point



The isoelectric point (pI) is the 2-D gel polyacrylamide gel electrophoresis.

For an pKas of this molecule.

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

For amino acids with more than two ionizable groups, such as List of standard amino acids).

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

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

isoelectric focusing.

At a 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.

Isoelectric Point of 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-OH+ 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 for various materials

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.[9] 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[7]). Lewis[7] also lists the IEP of barium titanate, BaTiO3 as being between pH 5 and 6, while Vamvakaki et al.[10] 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.

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 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. 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[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 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:

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 r 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. Delgado (editor), "Interfacial Electrokinetics and Electrophoresis", Marcel Dekker, 2002.
  7. ^ 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.
  8. ^ 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.
  9. ^ 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.
  10. ^ 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.
  11. ^ 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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