Editing Isoelectric point

{{Short description|pH at which a molecule carries no net electric charge}}

The '''isoelectric point''' ('''pI''', '''pH(I)''', '''IEP'''), is the [[pH]] at which a [[molecule]] carries no net [[electric charge|electrical charge]] or is electrically neutral in the [[statistical mean]]. The standard nomenclature to represent the isoelectric point is pH(I).<ref>Acceptable variants on pH(I) would include pH<sub>I</sub>, pH<sub>IEP</sub>, ''etc'';  the main point is that one cannot take the 'power' of I, rather one measures the pH subject to a nominated condition.</ref> However, pI is also used.<ref>{{GoldBookRef|title=isoelectric point ''in electrophoresis'' |file=I03275}}</ref> For [[concision|brevity]], this article uses pI.  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#In Physics and biochemistry|protons]] (H<sup>+</sup>).

Surfaces naturally charge to form a [[double layer (interfacial)|double layer]]. In the common case when the surface charge-determining ions are H<sup>+</sup>/HO<sup>−</sup>, 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 (chemistry)|solution]].  Biological [[amphoterism|amphoteric]] molecules such as [[protein]]s 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 by net charge in a [[polyacrylamide gel]] using either [[QPNC-PAGE|preparative native PAGE]], which uses a constant pH to separate proteins, or [[isoelectric focusing]], which uses a pH gradient to separate proteins. Isoelectric focusing is the first step in [[Two-dimensional gel electrophoresis|2-D polyacrylamide gel electrophoresis]].<ref>{{cite journal
|journal=Protein and Peptide Letters
|year=2007
|volume=14
|pages=389–93
|title=New hope for the diagnosis and therapy of Alzheimer's disease
|author=Kastenholz B
|doi=10.2174/092986607780363970
|pmid=17504097
|issue=4 }}
</ref>

In biomolecules, proteins can be separated by [[Ion chromatography|ion exchange chromatography]]. Biological proteins are made up of [[zwitterion]]ic amino acid compounds; the net charge of these proteins can be positive or negative depending on the pH of the environment. The specific pI of the target protein can be used to model the process around and the compound can then be purified from the rest of the mixture. Buffers of various pH can be used for this purification process to change the pH of the environment. When a mixture containing a target protein is loaded into an ion exchanger, the stationary matrix can be either positively-charged (for mobile anions) or negatively-charged (for mobile cations). At low pH values, the net charge of most proteins in the mixture is positive – in cation exchangers, these positively-charged proteins bind to the negatively-charged matrix. At high pH values, the net charge of most proteins is negative, where they bind to the positively-charged matrix in anion exchangers. When the environment is at a pH value equal to the protein's pI, the net charge is zero, and the protein is not bound to any exchanger, and therefore, can be eluted out.<ref>{{Cite journal|last=Dayton|first=W. R.|date=1983|title=Protein Separation Techniques|url=http://www.meatscience.org/docs/default-source/publications-resources/rmc/1983/protein-separation-techniques.pdf?sfvrsn=2|journal=Reciprocal Meat Conference Proceedings|volume=36|pages=98–102}}</ref>

== 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.<ref>For derivation of this expression see [[acid dissociation constant#Isoelectric point|acid dissociation constant]]</ref>

: <math> \mathrm{pI} = \frac{\mathrm{p}K_\mathrm{a1} + \mathrm{p}K_\mathrm{a2}}{2} </math>

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.

=== Examples ===
{|style="float: right; border: 1px solid #BBB; margin: .46em 0 0 .2em;"
|-
|[[File:Glycine pI-en.svg|250px]]

|[[File:AMP pI-en.svg|250px]]
|-
|align=center|glycine pK = 2.72, 9.60
|align=center|adenosine monophosphate pK = 0.9, 3.8, 6.1
|}
In the two examples (on the right) the isoelectric point is shown by the green vertical line. In [[glycine]] the pK values are separated by nearly 7 units. Thus in the gas phase, the concentration of the neutral species, glycine (GlyH), is effectively 100% of the analytical glycine concentration.<ref>{{Cite journal|last1=Jensen|first1=Jan H.|last2=Gordon|first2=Mark S.|date=August 1995|title=On the Number of Water Molecules Necessary To Stabilize the Glycine Zwitterion|url=https://pubs.acs.org/doi/pdf/10.1021/ja00136a013|journal=Journal of the American Chemical Society|volume=117|issue=31|pages=8159–8170|doi=10.1021/ja00136a013|issn=0002-7863}}</ref> Glycine may exist as a [[zwitterion]] at the isoelectric point, but the equilibrium constant for the isomerization reaction in solution
:<chem>H2NCH2CO2H <=> H3N+CH2CO2-</chem>
is not known.

The other example, [[adenosine monophosphate]] is shown to illustrate the fact that a third species may, in principle, be involved. In fact the concentration of {{chem2|(AMP)H3(2+)}} is negligible at the isoelectric point in this case.
If the pI is greater than the pH, the molecule will have a positive charge.

== Peptides and proteins ==

A number of algorithms for estimating isoelectric points of [[peptide]]s and [[protein]]s have been developed. Most of them use [[Henderson–Hasselbalch equation]] with different pK values. For instance, within the model proposed by Bjellqvist and co-workers, the pKs were determined between closely related immobilines by focusing the same sample in overlapping pH gradients.<ref>{{Cite journal|last1=Bjellqvist|first1=B.|last2=Hughes|first2=G. J.|last3=Pasquali|first3=C.|last4=Paquet|first4=N.|last5=Ravier|first5=F.|last6=Sanchez|first6=J. C.|last7=Frutiger|first7=S.|last8=Hochstrasser|first8=D.|date=1993-10-01|title=The focusing positions of polypeptides in immobilized pH gradients can be predicted from their amino acid sequences|journal=Electrophoresis|volume=14|issue=10|pages=1023–1031|issn=0173-0835|pmid=8125050|doi=10.1002/elps.11501401163|s2cid=38041111}}</ref> Some improvements in the methodology (especially in the determination of the pK values for modified amino acids) have been also proposed.<ref>{{Cite journal|last1=Gauci|first1=Sharon|last2=van Breukelen|first2=Bas|last3=Lemeer|first3=Simone M.|last4=Krijgsveld|first4=Jeroen|last5=Heck|first5=Albert J. R.|date=2008-12-01|title=A versatile peptide pI calculator for phosphorylated and N-terminal acetylated peptides experimentally tested using peptide isoelectric focusing|journal=Proteomics|volume=8|issue=23–24|pages=4898–4906|doi=10.1002/pmic.200800295|issn=1615-9861|pmid=19003858|s2cid=21527631}}</ref><ref>{{Cite journal|last1=Gasteiger|first1=Elisabeth|last2=Gattiker|first2=Alexandre|last3=Hoogland|first3=Christine|last4=Ivanyi|first4=Ivan|last5=Appel|first5=Ron D.|last6=Bairoch|first6=Amos|date=2003-07-01|title=ExPASy: the proteomics server for in-depth protein knowledge and analysis|journal=Nucleic Acids Research|volume=31|issue=13|pages=3784–3788|issn=0305-1048|pmc=168970|pmid=12824418|doi=10.1093/nar/gkg563}}</ref> More advanced methods take into account the effect of adjacent amino acids ±3 residues away from a charged [[Aspartic acid|aspartic]] or [[glutamic acid]], the effects on free C terminus, as well as they apply a correction term to the corresponding pK values using [[genetic algorithm]].<ref>{{Cite journal|last1=Cargile|first1=Benjamin J.|last2=Sevinsky|first2=Joel R.|last3=Essader|first3=Amal S.|last4=Eu|first4=Jerry P.|last5=Stephenson|first5=James L.|date=2008-07-01|title=Calculation of the isoelectric point of tryptic peptides in the pH 3.5–4.5 range based on adjacent amino acid effects|journal=Electrophoresis|volume=29|issue=13|pages=2768–2778|doi=10.1002/elps.200700701|issn=0173-0835|pmid=18615785|doi-access=free}}</ref> Other recent approaches are based on a [[Support vector machine|support vector machine algorithm]]<ref>{{Cite journal|last1=Perez-Riverol|first1=Yasset|last2=Audain|first2=Enrique|last3=Millan|first3=Aleli|last4=Ramos|first4=Yassel|last5=Sanchez|first5=Aniel|last6=Vizcaíno|first6=Juan Antonio|last7=Wang|first7=Rui|last8=Müller|first8=Markus|last9=Machado|first9=Yoan J.|date=2012-04-03|title=Isoelectric point optimization using peptide descriptors and support vector machines|journal=Journal of Proteomics|volume=75|issue=7|pages=2269–2274|doi=10.1016/j.jprot.2012.01.029|issn=1876-7737|pmid=22326964}}</ref> and pKa optimization against experimentally known protein/peptide isoelectric points.<ref>{{Cite journal  | last1 = Kozlowski | first1 = LP. | title = IPC - Isoelectric Point Calculator. | journal = Biol Direct | volume = 11 | issue = 1 | pages = 55 | year = 2016 | doi = 10.1186/s13062-016-0159-9 | pmid = 27769290 | pmc=5075173 | doi-access = free }}</ref>

Moreover, experimentally measured isoelectric point of proteins were aggregated into the databases.<ref name="pmid15274128">{{Cite journal  | last1 = Hoogland | first1 = C. | last2 = Mostaguir | first2 = K. | last3 = Sanchez | first3 = JC. | last4 = Hochstrasser | first4 = DF. | last5 = Appel | first5 = RD. | title = SWISS-2DPAGE, ten years later. | journal = Proteomics | volume = 4 | issue = 8 | pages = 2352–6 | year = 2004 | doi = 10.1002/pmic.200300830 | pmid = 15274128 | s2cid = 31933242 }}</ref><ref name="pmid25252779">{{Cite journal  | last1 = Bunkute | first1 = E. | last2 = Cummins | first2 = C. | last3 = Crofts | first3 = FJ. | last4 = Bunce | first4 = G. | last5 = Nabney | first5 = IT. | last6 = Flower | first6 = DR. | title = PIP-DB: the Protein Isoelectric Point database. | journal = Bioinformatics | volume = 31 | issue = 2 | pages = 295–6 | year = 2015 | doi = 10.1093/bioinformatics/btu637 | pmid = 25252779 | doi-access = free }}</ref> Recently, a database of isoelectric points for all proteins predicted using most of the available methods had been also developed.<ref name="pmid27789699">{{Cite journal  | last1 = Kozlowski | first1 = LP. | title = Proteome-pI: proteome isoelectric point database. | journal = Nucleic Acids Res | year = 2016 | doi = 10.1093/nar/gkw978 | pmid = 27789699 | pmc=5210655 | volume=45| issue = D1 | pages = D1112–D1116 }}</ref>

In practice, a protein with an excess of basic aminoacids (arginine, lysine and/or histidine) will bear an isoelectric point roughly greater than 7 (basic), while a protein with an excess of acidic aminoacids (aspartic acid and/or glutamic acid) will often have an isoelectric point lower than 7 (acidic).
The electrophoretic linear (horizontal) separation of proteins by Ip along a pH gradient in a polyacrylamide gel (also known as [[isoelectric focusing]]), followed by a standard molecular weight linear (vertical) separation in a second polyacrylamide gel ([[SDS-PAGE]]), constitutes the so called [[two-dimensional gel electrophoresis]] or PAGE 2D. This technique allows a thorough separation of proteins as distinct "spots", with proteins of high molecular weight and low Ip migrating to the upper-left part of the bidimensional gel, while proteins with low molecular weight and high Ip locate to the bottom-right region of the same gel.

== Ceramic materials ==
The isoelectric points (IEP) of metal oxide ceramics are used extensively in material science in various aqueous processing steps (synthesis, modification, etc.).  In the absence of chemisorbed or physisorbed species particle surfaces in aqueous suspension are generally assumed to be covered with surface hydroxyl species, M-OH (where M is a metal such as Al, Si, etc.).<ref name="ref2pineapple">
{{cite journal
| last1=Hanaor
| first1=D.A.H.
| last2=Michelazzi
| first2=M.
| last3=Leonelli
| first3=C.
| last4=Sorrell
| first4=C.C.
| title= The effects of carboxylic acids on the aqueous dispersion and electrophoretic deposition of ZrO<sub>2</sub>
| journal= Journal of the European Ceramic Society
| year=2012
| volume=32
| issue=1
| pages=235–244
| doi=10.1016/j.jeurceramsoc.2011.08.015| arxiv=1303.2754
| s2cid=98812224
}}</ref> At pH values above the IEP, the predominant surface species is M-O<sup>−</sup>, while at pH values below the IEP, M-OH<sub>2</sub><sup>+</sup> species predominate. Some approximate values of common ceramics are listed below:<ref>{{cite journal | last1 = Haruta | first1 = M | year = 2004 | title = Nanoparticulate Gold Catalysts for Low-Temperature CO Oxidation | journal = Journal of New Materials for Electrochemical Systems | volume = 7 | pages = 163–172 }}</ref><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>
{| class="wikitable sortable"
|-
! Material
! IEP
|-
| [[tungsten(VI) oxide|WO<sub>3</sub>]]<ref name="Kosmulski"/>
| 0.2–0.5
|-
| [[antimony(V) oxide|Sb<sub>2</sub>O<sub>5</sub>]]<ref name="Kosmulski"/>
| <0.4–1.9
|-
| [[vanadium(V) oxide|V<sub>2</sub>O<sub>5</sub>]]<ref name="Kosmulski"/><ref name="Jolivet"/>
| 1–2 (3)
|-
| [[manganese(IV) oxide|δ-MnO<sub>2</sub>]]
| 1.5
|-
| [[silicon dioxide|SiO<sub>2</sub>]]<ref name="Kosmulski"/>
| 1.7–3.5
|-
| [[silicon carbide|SiC]]<ref>U.S. Patent 5,165,996</ref>
| 2–3.5
|-
| [[tantalum(V) oxide|Ta<sub>2</sub>O<sub>5</sub>]]<ref name="Kosmulski"/>
| 2.7–3.0
|-
| [[titanium(IV) oxide|TiO<sub>2</sub>]]<ref name=epd>[https://arxiv.org/ftp/arxiv/papers/1303/1303.2742.pdf Anodic Aqueous Electrophoretic Deposition of Titanium Dioxide Using Carboxylic Acids as Dispersing Agents] Journal of the European Ceramic Society, 31(6), 1041-1047, 2011</ref>
| 2.8–3.8
|-
| γ-[[Iron (III) oxide|Fe<sub>2</sub>O<sub>3</sub>]]<ref name="Kosmulski"/>
| 3.3–6.7
|-
| [[tin(IV) oxide|SnO<sub>2</sub>]]<ref name="Lewis">{{cite journal | last1 = Lewis | first1 = JA | year = 2000 | title = Colloidal Processing of Ceramics | doi = 10.1111/j.1151-2916.2000.tb01560.x | journal = Journal of the American Ceramic Society | volume = 83 | issue = 10| pages = 2341–2359 | citeseerx = 10.1.1.514.1543 | s2cid = 9513223 }}</ref>
|  4–5.5 (7.3)
|-
| [[zirconium(IV) oxide|ZrO<sub>2</sub>]]<ref name="Kosmulski"/>
| 4–11
|-
| [[indium tin oxide|ITO]]<ref>{{cite journal | last1 = Daido | first1 = T | last2 = Akaike | first2 = T | year = 1993 | title = Electrochemistry of cytochrome c: influence of coulombic attraction with indium tin oxide electrode | journal = Journal of Electroanalytical Chemistry | volume = 344 | issue = 1–2| pages = 91–106 | doi=10.1016/0022-0728(93)80048-m}}</ref>
| 6
|-
| [[chromium(III) oxide|Cr<sub>2</sub>O<sub>3</sub>]]<ref name="Kosmulski"/><ref name="Jolivet"/> 
| 6.2–8.1 (7)
|-
| [[magnetite|Fe<sub>3</sub>O<sub>4</sub>]]<ref name="Kosmulski"/>
| 6.5–6.8
|-
| [[cerium(IV) oxide|CeO<sub>2</sub>]]<ref name="Kosmulski"/>
| 6.7–8.6
|-
| [[yttrium(III) oxide|Y<sub>2</sub>O<sub>3</sub>]]<ref name="Kosmulski"/>
| 7.15–8.95
|-
| γ-[[aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]] 
| 7–8
|-
| β-MnO<sub>2</sub><ref name="Jolivet"/>
| 7.3
|-
| [[thallium(I) oxide|Tl<sub>2</sub>O]]<ref>{{cite journal | last1 = Kosmulski | first1 = M | last2 = Saneluta | first2 = C | year = 2004 | title = Point of zero charge/isoelectric point of exotic oxides: Tl2O3 | journal = Journal of Colloid and Interface Science | volume = 280 | issue = 2| pages = 544–545 | doi=10.1016/j.jcis.2004.08.079| pmid = 15533430 | bibcode = 2004JCIS..280..544K }}</ref>
| 8
|-
| α-[[aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]]
| 8–9
|-
| α-Fe<sub>2</sub>O<sub>3</sub><ref name="Kosmulski"/>
| 8.4–8.5
|-
| [[zinc oxide|ZnO]]<ref name="Kosmulski"/>
| 8.7–10.3
|-
| [[silicon nitride|Si<sub>3</sub>N<sub>4</sub>]]<ref name="Lewis"/>
| 9
|-
| [[copper(II) oxide|CuO]]<ref name="Lewis"/>
| 9.5
|-
| [[lanthanum(III) oxide|La<sub>2</sub>O<sub>3</sub>]]
| 10
|-
| [[nickel(II) oxide|NiO]]<ref name="Lewis"/>
| 10–11
|-
| [[lead(II) oxide|PbO]]<ref name="Kosmulski">Marek Kosmulski, "Chemical Properties of Material Surfaces", Marcel Dekker, 2001.</ref>
| 10.7–11.6
|-
| [[magnesium oxide|MgO]]<ref name="Kosmulski"/>
| 12–13 (9.8·12.7)
|-
|}
''Note: The following list gives the isoelectric point at 25&nbsp;°C for selected materials in water. The exact value can vary widely, depending on material factors such as purity and phase as well as physical parameters such as temperature. Moreover, the precise measurement of isoelectric points can be difficult, thus many sources often cite differing values for isoelectric points of these materials.''

Mixed oxides may exhibit isoelectric point values that are intermediate to those of the corresponding pure oxides. For example, a synthetically prepared amorphous [[aluminosilicate]] (Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>) was initially measured as having IEP of 4.5 (the electrokinetic behavior of the surface was dominated by surface Si-OH species, thus explaining the relatively low IEP value).<ref>{{cite journal | last1 = Jara | first1 = A.A. | last2 = Goldberg | first2 = S. | last3 = Mora | first3 = M.L. | year = 2005 | title = Studies of the surface charge of amorphous aluminosilicates using surface complexation models | journal = Journal of Colloid and Interface Science | volume = 292 | issue = 1| pages = 160–170 | doi=10.1016/j.jcis.2005.05.083| pmid = 16051258 | bibcode = 2005JCIS..292..160J | hdl = 10533/176403 | hdl-access = free }}</ref> 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.<ref name="Lewis"/> Similarly, also IEP of [[barium titanate]], BaTiO<sub>3</sub> was reported in the range 5–6<ref name="Lewis"/> while others got a value of 3.<ref name="VamvakakiBillingham2001">{{cite journal|last1=Vamvakaki|first1=Maria|last2=Billingham|first2=Norman C.|last3=Armes|first3=Steven P.|last4=Watts|first4=John F.|last5=Greaves|first5=Stephen J.|title=Controlled structure copolymers for the dispersion of high-performance ceramics in aqueous media|journal=Journal of Materials Chemistry|volume=11|issue=10|year=2001|pages=2437–2444|issn=0959-9428|doi=10.1039/b101728o}}</ref> Mixtures of [[Titanium dioxide|titania]] (TiO<sub>2</sub>) and [[zirconia]] (ZrO<sub>2</sub>) were studied and found to have an isoelectric point between 5.3–6.9, varying non-linearly with %(ZrO<sub>2</sub>).<ref name="GlennaLDrisko2009">{{cite journal|last1=Drisko|first1=Glenna L|last2=Luca|first2=Vittorio|last3=Sizgek|first3=Erden|last4=Scales|first4=Nicolas F.|last5=Caruso|first5=Rachel A.|title=Template Synthesis and Adsorption Properties of Hierarchically Porous Zirconium Titanium Oxides|journal=Langmuir|volume=25|issue=9|year=2009|pages=5286–5293|issn=0743-7463|doi=10.1021/la804030h|pmid=19397363}}</ref> The surface charge of the mixed oxides was correlated with acidity. Greater titania content led to increased Lewis acidity, whereas zirconia-rich oxides displayed Br::onsted acidity. The different types of acidities produced differences in ion adsorption rates and capacities.

== 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>&minus;</sup> 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 [[Electrical 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 from 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.<ref name="ref2pineapple" /> In this context, specific adsorption is understood as adsorption occurring in a [[Double layer (interfacial)|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 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, p''K''<sup>&minus;</sup> and p''K''<sup>+</sup> to define the two conditions in terms of the relative number of charged sites:

:<math> \mathrm{p}K^- - \mathrm{p}K^+ = \Delta \mathrm{p}K = \log {\frac{\left[\mathrm{MOH}\right]^2}{\left[\mathrm{MOH}{_2^+}\right]\left[\mathrm{MO}^-\right]}} </math>

For large Δp''K'' (>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 Δp''K'', there are many charged species in approximately equal numbers, so one speaks of the IEP.

== See also ==
* [[Electrophoretic deposition]]
* [[Henderson-Hasselbalch equation]]
* [[Isoelectric focusing]]
* [[Isoionic point]]
* [[PKa|pK acid dissociation constant]]
* [[QPNC-PAGE|Preparative native PAGE]]
* [[Zeta potential]]

== References ==
{{reflist|30em}}

== 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 ==
* [http://isoelectric.org IPC – Isoelectric Point Calculator ] &mdash; calculate protein isoelectric point using over 15 methods
* [https://www.protpi.ch/Calculator/ProteinTool prot pi – protein isoelectric point] &mdash; an online program for calculating pI of proteins (include multiple subunits and posttranslational modifications)
* [http://www2.iq.usp.br/docente/gutz/Curtipot_.html CurTiPot] &mdash;  a suite of spreadsheets for computing acid-base equilibria (charge versus pH plot of amphoteric molecules e.g., amino acids)
* [https://github.com/EBjerrum/pICalculax pICalculax] &mdash; Isoelectric point (pI) predictor for chemically modified peptides and proteins
* [http://world-2dpage.expasy.org/swiss-2dpage/ SWISS-2DPAGE] {{Webarchive|url=https://web.archive.org/web/20161210141012/http://world-2dpage.expasy.org/swiss-2dpage/ |date=2016-12-10 }} &mdash; a database of isoelectric points coming from two-dimensional polyacrylamide gel electrophoresis (~ 2,000 proteins)
* [http://www.pip-db.org PIP-DB] &mdash; a Protein Isoelectric Point database (~ 5,000 proteins)
* [http://isoelectricpointdb.org ''Proteome-pI''] &mdash; a proteome isoelectric point database (predicted isoelectric point for all proteins)

{{DEFAULTSORT:Isoelectric Point}}
[[Category:Ions]]
[[Category:Molecular biology]]