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Isoelectric point

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Isoelectric point

The isoelectric point (pI, pH(I), IEP), is the pH at which a particular molecule carries no net electrical charge. The standard nomenclature to represent the isoelectric point is pH(I),[1] although pI is also commonly seen,[2] and is used in this article for brevity. The net charge on the molecule is affected by pH of its surrounding environment and can become more positively or negatively charged due to the gain or loss, respectively, of protons (H+).

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.

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 that 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 that 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 either QPNC-PAGE or 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 1
    • Examples 1.1
  • Ceramic materials 2
    • Examples of isoelectric points 2.1
  • Isoelectric point versus point of zero charge 3
  • See also 4
  • References 5
  • Further reading 6
  • External links 7

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 pKas of this molecule.[3]

pI = }

For large ΔpK (>4 according to Jolivet), the predominant 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.

See also

References

  1. ^ Acceptable variants on pH(I) would include pHI, pHIEP, etc; the main point is that one cannot take the 'power' of I, rather one measures the pH subject to a nominated condition.
  2. ^ IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook. Last update: 2014-02-24; version: 2.3.3. DOI of this term: doi:10.1351/goldbook.I03275.
  3. ^ For derivation of this expression see acid dissociation constant
  4. ^ a b Hanaor, D.A.H.; Michelazzi, M.; Leonelli, C.; Sorrell, C.C. (2012). "2"The effects of carboxylic acids on the aqueous dispersion and electrophoretic deposition of ZrO. Journal of the European Ceramic Society 32 (1): 235–244.  
  5. ^ Haruta M (2004). 'Nanoparticulate Gold Catalysts for Low-Temperature CO Oxidation', Journal of New Materials for Electrochemical Systems, vol. 7, pp 163–172.
  6. ^ vol. 50, pp. 1211-1229.Pure and Applied ChemistryBrunelle JP (1978). 'Preparation of Catalysts by Metallic Complex Adsorption on Mineral Oxides'.
  7. ^ 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.
  8. ^ 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).
  9. ^ U.S. Patent 5,165,996
  10. ^ 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.
  11. ^ Daido T and Akaike T (1993). 'Electrochemistry of cytochrome c: influence of coulombic attraction with indium tin oxide electrode', Journal of Electroanalytical Chemistry vol. 344, no. 1-2, pp. 91–106.
  12. ^ 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.
  13. ^ 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.
  14. ^ , vol. 11, pp. 2437-2444.Journal of Materials ChemistryVamvakaki, 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',
  15. ^ 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 0-7167-4339-6
  • Kosmulski M. (2009). Surface Charging and Points of Zero Charge. CRC Press; 1st edition (Hardcover). ISBN 978-1-4200-5188-9

External links

  • Calculation of protein isoelectric point — free online and offline program to calculation pI and more theoretical information about this subject.
  • Isoelectric point determination and Charge versus pH plot of amphoteric molecules (e.g., amino acids) by a free suite of spreadsheets for computing acid-base equilibria.
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