Isoelectric point

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The isoelectric point (pI) is the pH at which a molecule or surface carries no net electrical charge. In order to have a sharp isoelectric point, a molecule (or surface) must be amphoteric, meaning it must have both acidic and basic functional groups. Proteins and amino acids are common molecules that meet this requirement.

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

For amino acids with more than two ionizable groups, such as lysine for example, 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 amide 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 amide pKa values (found at List of standard amino acids).

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

Proteins can be separated according to their isoelectric point in a process known as isoelectric focusing.

At a pH below the pI, proteins carry a net positive charge. Above the pI they carry a net negative charge. This has implications for running electrophoretic gels (see Agarose gel electrophoresis). 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.

Contents 1 Isoelectric Point of Ceramic Materials 2 Isoelectric Point versus Point of Zero Charge 3 References 4 External links

[edit] 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 2004 and Brunelle 1978). 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.

antimony oxide: <1

tungsten oxide: <1

silicon oxide (silica): 1-3

zirconium oxide (zirconia): 4-7

tin oxide: 4-5

manganese oxide: 4-5

titanium oxide (titania): 6

gamma iron oxide: 7

cerium oxide (ceria): 7

gamma aluminum oxide (gamma alumina): 7-8

thallium oxide: 8 (Kosmulski et al. 2004)

alpha iron oxide: 8-9

alpha aluminum oxide (alpha alumina): 8-9

yttrium oxide (yttria): 9

zinc oxide: 9-10

lanthanum oxide: 10

magnesium oxide (magnesia): 12-13

[edit] 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 is important to make the distinction. 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 (see, for example, Jolivet 2000). 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⁺ = ΔpK = log [MOH]²/[MOH₂⁺][MO^-]

For large ΔpK (>4 according to Jolivet), the predominate species is MOH while there are relatively few charged species - so the PZC is relavent. For small values of ΔpK, there are many charged species in approximately equal numbers, so one speaks of the IEP.

[edit] References

• Nelson DL, Cox MM (2004). Lehninger Principles of Biochemistry. W. H. Freeman; 4th edition (Hardcover). ISBN 0-7167-4339-6
• Haruta M (2004). 'Nanoparticulate Gold Catalysts for Low-Temperature CO Oxidation', Journal of New Materials for Electrochemical Systems, vol. 7, pp 163-172.
• Brunelle JP (1978). 'Preparation of Catalysts by Metallic Complex Adsorption on Mineral Oxides'. Pure and Applied Chemistry vol. 50, pp. 1211-1229.
• 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.
• Jolivet JP (2000). Metal Oxide Chemistry and Synthesis, John Wiley & Sons Ltd. ISBN 0-471-97056-5 (English translation of the original French text, De la Solution à l'Oxyde, InterEditions et CNRS Editions, Paris, 1994).

[edit] External links

• EMBL WWW Gateway to Isoelectric Point Service — calculates the pI for an input amino acid sequence.
• Calculation of protein isoelectric point — free online and offline program to calculation pI and more theoretical information about this subject