Isoelectric point
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Intro copyedits.
The '''isoelectric point''' (pI) is the [[pH]] at which a particular [[molecule]] or surface carries no net [[electric charge|electrical charge]]. [[Amphoteric]] molecules called [[zwitterion]]s contain both positive and negative charges depending on the [[functional groups]] present in the molecule. They are affected by pH of their surrounding environment and can become more positively or negatively charged due to the loss or gain of [[protons#In Physics and biochemistry|protons]] (H<sup>+</sup>).
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 [[protein]]s 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 [[Two-dimensional gel electrophoresis|2-D gel polyacrylamide gel electrophoresis]].
== Calculating pI values ==
For an [[amino acid]] with only one [[amine]] and one [[carboxyl]] group, the pI can be calculated from the [[pKa]]s of this molecule.
: <math> pI = {{pK_1 + pK_2} \over 2} </math>
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]]).
: <math> pI = {{9.06 + 10.54} \over 2} = 9.80 </math>
However, a more exact treatment of this requires advanced [[acid]]/[[Base (chemistry)|base]] knowledge and calculations.
The [[pH]] of an electrophoretic gel is determined by the [[Buffer solution|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<sup>-</sup>, while at pH values below the IEP, M-OH<sup>+</sup> species predominate. Some approximate values of common ceramics are listed below (Haruta<ref>Haruta M (2004). 'Nanoparticulate Gold Catalysts for Low-Temperature CO Oxidation', ''Journal of New Materials for Electrochemical Systems'', vol. 7, pp 163-172.</ref> and Brunelle<ref>[http://www.iupac.org/publications/pac/1978/pdf/5009x1211.pdf Brunelle JP (1978). 'Preparation of Catalysts by Metallic Complex Adsorption on Mineral Oxides'. ''Pure and Applied Chemistry'' vol. 50, pp. 1211-1229.]</ref>, 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 pH<sub>25°C</sub> of isoelectric point at 25 °C for selected materials in water:
''Note: The list is ordered by increasing pH values.''
*[[tungsten(VI) oxide]] WO<sub>3</sub>: 0.2-0.5 <ref name="Kosmulski"/>
*[[antimony(V) oxide]] Sb<sub>2</sub>O<sub>5</sub>: <0.4 to 1.9 <ref name="Kosmulski"/>
*[[vanadium(V) oxide]] (vanadia) V<sub>2</sub>O<sub>5</sub>: 1-2 <ref name="Jolivet"/> (3 <ref name="Kosmulski"/>)
*[[silicon oxide]] (silica) SiO<sub>2</sub>: 1.7-3.5 <ref name="Kosmulski"/>
*[[silicon carbide]] (alpha) SiC: 2-3.5 <ref>U.S. Patent 5,165,996</ref>
*[[tantalum(V) oxide]], Ta<sub>2</sub>O<sub>5</sub>: 2.7-3.0 <ref name="Kosmulski"/>
*[[tin(IV) oxide]] SnO<sub>2</sub>: 4-5.5 (7.3 <ref name="Lewis">Lewis, JA (2000). 'Colloidal Processing of Ceramics', ''Journal of the American Ceramic Society'' vol. 83, no. 10, pp.2341-2359.</ref>)
*[[zirconium(IV) oxide]] (zirconia) ZrO<sub>2</sub>: 4-11 <ref name="Kosmulski"/>
*[[manganese(IV) oxide]] MnO<sub>2</sub>: 4-5
*delta-MnO<sub>2</sub> 1.5, beta-MnO<sub>2</sub> 7.3 <ref name="Jolivet"/>
*[[titanium(IV) oxide]] (titania) ([[rutile]] or [[anatase]]) TiO<sub>2</sub>: 3.9-8.2 <ref name="Kosmulski"/>
*[[silicon nitride]] Si<sub>3</sub>N<sub>4</sub>: 6-7
*[[magnetite|iron (II, III) oxide]] (magnetite) Fe<sub>3</sub>O<sub>4</sub>: 6.5-6.8 <ref name="Kosmulski"/>
*[[maghemite|gamma iron (III) oxide]] (maghemite) Fe<sub>2</sub>O<sub>3</sub>: 3.3-6.7 <ref name="Kosmulski"/>
*[[cerium(IV) oxide]] (ceria) CeO<sub>2</sub>: 6.7-8.6 <ref name="Kosmulski"/>
*[[chromium(III) oxide]] (chromia) Cr<sub>2</sub>O<sub>3</sub>: 7 <ref name="Jolivet"/> (6.2-8.1 <ref name="Kosmulski"/>)
*gamma [[aluminium oxide]] (gamma alumina) Al<sub>2</sub>O<sub>3</sub>: 7-8
*[[thallium(I) oxide]] Tl<sub>2</sub>O: 8 <ref>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.</ref>
*[[hematite|alpha iron (III) oxide]] (hematite) Fe<sub>2</sub>O<sub>3</sub>: 8.4-8.5 <ref name="Kosmulski"/>
*alpha [[aluminium oxide]] (alpha alumina, corundum) Al<sub>2</sub>O<sub>3</sub>: 8-9
*[[silicon nitride]] Si<sub>3</sub>N<sub>4</sub>: 9 <ref name="Lewis"/>
*[[yttrium(III) oxide]] (yttria) Y<sub>2</sub>O<sub>3</sub>: 7.15-8.95 <ref name="Kosmulski"/>
*[[copper(II) oxide]] CuO: 9.5 <ref name="Lewis"/>
*[[zinc oxide]] ZnO: 8.7-10.3 <ref name="Kosmulski"/>
*[[lanthanum(III) oxide]] La<sub>2</sub>O<sub>3</sub>: 10
*[[nickel(II) oxide]] NiO: 10-11 <ref name="Lewis"/> (9.9-11.3 <ref name="Kosmulski"/>)
*[[lead(II) oxide]] PbO: 10.7-11.6 <ref name="Kosmulski">Marek Kosmulski, "Chemical Properties of Material Surfaces", Marcel Dekker, 2001.</ref>
*[[magnesium oxide]] (magnesia) MgO: 12-13 (9.8-12.7 <ref name="Kosmulski"/>)
Mixed oxides may exhibit isoelectric point values that are intermediate to those of the corresponding pure oxides. For example, Jara ''et al.''<ref>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.</ref> measured an IEP of 4.5 for a synthetically-prepared amorphous aluminosilicate (Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>). 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 3Al<sub>2</sub>O<sub>3</sub>-2SiO<sub>2</sub> by others (see Lewis<ref name="Lewis"/>). Lewis<ref name="Lewis"/> also lists the IEP of barium titanate, BaTiO<sub>3</sub> as being between pH 5 and 6, while Vamvakaki et al.<ref>[http://www.rsc.org/ej/JM/2001/b101728o.pdf 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.]</ref> 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 TiO<sub>2</sub>-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<sup>+</sup>/OH<sup>-</sup> 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 [[double layer|shear plane]]. This is termed the isoelectric point<ref>A.W. Adamson, A.P. Gast, "Physical Chemistry of Surfaces", John Wiley and Sons, 1997.</ref>. 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 [[adsorption|specifically adsorbed]] positive or negative charges. In this context, specific adsorption is understood as adsorption occurring the [[double layer|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<ref name="Jolivet">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).</ref>, 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<sup>-</sup> and pK<sup>+</sup> to define the two conditions in terms of the relative number of charged sites:
:<math> pK^- - pK^+ = \Delta pK = \log {\frac{\left[MOH\right]^2}{\left[MOH{_2^+}\right]\left[MO^-\right]}} </math>
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 ==
{{reflist}}
== Further reading==
* Nelson DL, Cox MM (2004). ''Lehninger Principles of Biochemistry''. W. H. Freeman; 4th edition (Hardcover). ISBN 0-7167-4339-
== External links ==
*[http://www.embl-heidelberg.de/cgi/pi-wrapper.pl EMBL WWW Gateway to Isoelectric Point Service] — calculates the pI for an input amino acid sequence.
*[http://isoelectric.ovh.org Calculation of protein isoelectric point] — free online and offline program to calculation pI and more theoretical information about this subject.
[[Category:Ions]]
[[Category:Molecular biology]]
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