The
isoelectric point (
pI), sometimes abbreviated to
IEP, is the
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
at which a particular
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
or surface carries no net
electrical chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
.
Amphoteric molecules called
zwitterionA zwitterion is a chemical compound that carries a total net charge of 0 and is thus electrically neutral, but carries formal charges on different atoms....
s contain both positive and negative charges depending on the functional groups present in the molecule. The net charge on the molecule is affected by pH of their surrounding environment and can become more positively or negatively charged due to the loss or gain of protons (H
+). The pI is the pH value at which the molecule carries no electrical charge or the negative and positive charges are equal.
Surfaces naturally charge to form a
double layerA double layer is a structure that appears on the surface of an object when it is placed into a liquid. This object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. It consists of two parallel layers of ions...
. 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. Again, the pI is the pH value of the solution at which the surfaces carries no net charge.
The pI value can affect the solubility of a molecule at a given pH. Such molecules have minimum
solubilitySolubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a liquid solvent to form a homogeneous solution. The solubility of a substance strongly depends on the used solvent as well as on temperature and pressure...
in water or salt solutions at the pH which corresponds to their
pI and often precipitate out of
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
. Biological amphoteric molecules such as
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
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
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
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 gelA Polyacrylamide Gel is a separation matrix used in electrophoresis of biomolecules, such as proteins or DNA fragments. Traditional DNA sequencing techniques such as Maxam-Gilbert or Sanger methods used polyacrylamide gels to separate DNA fragments differing by a single base-pair in length so the...
using a technique called
isoelectric focusing-Isoelectric focusing in laboratory:Isoelectric focusing , also known as electrofocusing, is a technique for separating different molecules by their electric charge differences. It is a type of zone electrophoresis, usually performed in a gel, that takes advantage of the fact that a molecule's...
, which uses a pH gradient to separate proteins. Isoelectric focusing is also the first step in
2-D gel polyacrylamide gel electrophoresisTwo-dimensional gel electrophoresis, abbreviated as 2-DE or 2-D electrophoresis, is a form of gel electrophoresis commonly used to analyze proteins...
.
Calculating pI values
For an
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
with only one
amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
and one carboxyl group, the pI can be calculated from the
meanIn statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....
of the
pKaPKA or pKa may be:* Protein kinase A, a cAMP activated protein kinase* pKa, the symbol for Acid dissociation constant* Pi Kappa Alpha, the North-American social fraternity* Public Key Authentication...
's of this molecule.
For amino acids with more than two ionizable groups, such as
lysineLysine is an α-amino acid with the chemical formula HO
2CCH
4NH
2. This amino acid is an essential amino acid, which means that humans cannot synthesize it. Its codons are AAA and AAG.Lysine is a base, as are arginine and histidine...
, 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.
LysineLysine is an α-amino acid with the chemical formula HO
2CCH
4NH
2. This amino acid is an essential amino acid, which means that humans cannot synthesize it. Its codons are AAA and AAG.Lysine is a base, as are arginine and histidine...
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).
However, a more exact treatment of this requires advanced
acidAn acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0...
/
baseIn chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. Bases are also the oxides or hydroxides of metals. A soluble base is also often referred to as an alkali if hydroxide ions are involved. This refers to the Brønsted-Lowry theory of acids and bases...
knowledge and calculations.
The
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
of an electrophoretic gel is determined by the
bufferA buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of acid or base is added to it. Buffer solutions are used as a means...
used for that gel. If the
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
of the buffer is above the pI of the protein being run, the
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
will migrate to the positive pole (negative charge is attracted to a positive pole). If the
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
of the buffer is below the pI of the
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
being run, the
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
will migrate to the negative pole of the gel (positive charge is attracted to the negative pole). If the
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
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
-, while at pH values below the IEP, M-OH
2+ species predominate. Some approximate values of common ceramics are listed below (Haruta and Brunelle, 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
25°C of isoelectric point at 25 °C for selected materials in water:
Note: The list is ordered by increasing pH values.
- tungsten(VI) oxide WO3: 0.2-0.5
- antimony(V) oxide Sb2O5: <0.4 to 1.9
- vanadium(V) oxide
Vanadium oxide is the chemical compound with the formula V2O5. Commonly known as vanadium pentoxide, this orange solid is the most important compound of vanadium. Upon heating it reversibly loses oxygen...
(vanadia) V2O5: 1-2 (3 )
- silicon oxide (silica) SiO2: 1.7-3.5
- silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with a chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
(alpha) SiC: 2-3.5
- tantalum(V) oxide, Ta2O5: 2.7-3.0
- tin(IV) oxide SnO2: 4-5.5 (7.3 )
- zirconium(IV) oxide (zirconia) ZrO2: 4-11
- manganese(IV) oxide
Manganese dioxide is the inorganic compound with the formula MnO2. This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese. It is also present in manganese nodules. The principal use for MnO2 is for dry-cell batteries, such...
MnO2: 4-5
- delta-MnO2 1.5, beta-MnO2 7.3
- titanium(IV) oxide (titania) (rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO
2.Rutile is the most common natural form of TiO
2. Two rarer polymorphs of TiO
2 are known:...
or anataseAnatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but,...
) TiO2: 3.9-8.2
- silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. It is a hard ceramic having high strength over a broad temperature range, moderate thermal conductivity, low coefficient of thermal expansion, moderately high elastic modulus, and unusually high fracture toughness for a ceramic...
Si3N4: 6-7
- iron (II, III) oxide
Magnetite is a ferrimagnetic mineral with chemical formula Fe
3O
4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name ferrous-ferric oxide...
(magnetite) Fe3O4: 6.5-6.8
- gamma iron (III) oxide
Maghemite is a member of the family of iron oxides. It has the same structure as magnetite, that is, it is spinel ferrite and is also ferrimagnetic...
(maghemite) Fe2O3: 3.3-6.7
- cerium(IV) oxide
Cerium oxide, also known as ceric oxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare earth metal cerium...
(ceria) CeO2: 6.7-8.6
- chromium(III) oxide
Chromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment...
(chromia) Cr2O3: 7 (6.2-8.1 )
- gamma aluminium oxide
Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula
23. It is also commonly referred to as alumina, corundum, sapphire, ruby or aloxite in the mining, ceramic and materials science communities. It is produced by the Bayer process from bauxite...
(gamma alumina) Al2O3: 7-8
- thallium(I) oxide Tl2O: 8
- alpha iron (III) oxide
Hematite, also spelled as hæmatite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
(hematite) Fe2O3: 8.4-8.5
- alpha aluminium oxide
Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula
23. It is also commonly referred to as alumina, corundum, sapphire, ruby or aloxite in the mining, ceramic and materials science communities. It is produced by the Bayer process from bauxite...
(alpha alumina, corundum) Al2O3: 8-9
- silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. It is a hard ceramic having high strength over a broad temperature range, moderate thermal conductivity, low coefficient of thermal expansion, moderately high elastic modulus, and unusually high fracture toughness for a ceramic...
Si3N4: 9
- yttrium(III) oxide
Yttrium oxide is Y2O3. It is an air-stable, white solid substance. Yttrium oxide is used as a common starting material for both materials science as well as inorganic compounds.-In materials science:...
(yttria) Y2O3: 7.15-8.95
- copper(II) oxide
Copper oxide or cupric oxide is the higher oxide of copper. As a mineral, it is known as tenorite.- Chemistry :It is a black solid with an ionic structure which melts above 1200 °C with some loss of oxygen...
CuO: 9.5
- zinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It usually appears as a white powder, nearly insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber Zinc oxide is an inorganic compound with the...
ZnO: 8.7-10.3
- lanthanum(III) oxide
Lanthanum oxide is La2O3, a chemical compound containing the rare earth element lanthanum and oxygen. It is used to develop ferroelectric materials, and in optical materials...
La2O3: 10
- nickel(II) oxide
Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel although Nickel(II) oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel although Nickel(II) oxide is the...
NiO: 10-11 (9.9-11.3 )
- lead(II) oxide
Lead oxide is the chemical compound with the formula PbO. Lead oxide occurs in two forms: red, having a tetragonal crystal structure and yellow, having an orthorhombic crystal structure...
PbO: 10.7-11.6
- magnesium oxide
Magnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of . It is formed by an ionic bond between one magnesium and one oxygen atom. Magnesium oxide is hygroscopic in nature and care must be taken to...
(magnesia) MgO: 12-13 (9.8-12.7 )
Mixed oxides may exhibit isoelectric point values that are intermediate to those of the corresponding pure oxides. For example, Jara
et al. measured an IEP of 4.5 for a synthetically-prepared amorphous aluminosilicate (Al
2O
3-SiO
2). 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
2O
3-2SiO
2 by others (see Lewis). Lewis also lists the IEP of barium titanate, BaTiO
3 as being between pH 5 and 6, while Vamvakaki et al. 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
2-rich surfaces.
The farther the pH of an Amino Acid solution is from its pl the greater the electric charge on that population of molecules.
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 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 phenomenaElectrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids or in porous bodies filled with fluid. The term heterogeneous here means a fluid containing particles...
generally measure
zeta potentialZeta potential is an abbreviation for electrokinetic potential in colloidal systems. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential...
, and a zero zeta potential is interpreted as the point of zero net charge at the
shear planeThe term double layer is used in different scientific disciplines:* Double layer , in plasma physics* Double layer , in Interface and Colloid Science* Double layer , in Electrochemistry* Double layer , in Biology...
. This is termed the isoelectric point. 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
specifically adsorbedAdsorption is the accumulation of atoms or molecules on the surface of a material. This process creates a film of the adsorbate on the adsorbent's surface. It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution...
positive or negative charges. In this context, specific adsorption is understood as adsorption occurring the
Stern layerThe term double layer is used in different scientific disciplines:* Double layer , in plasma physics* Double layer , in Interface and Colloid Science* Double layer , in Electrochemistry* Double layer , in Biology...
or
chemisorptionChemisorption is a classification of adsorption characterized by a strong interaction between an adsorbate and a substrate surface, as opposed to physisorption which is characterized by a weak Van der Waals force. A distinction between the two can be difficult and it is conventionally accepted...
. 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, 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 the quantity of charged sites at the point of net zero charge. Jolivet uses the intrinsic surface equilibrium constants, pK
- and pK
+ to define the two conditions in terms of the relative number of charged sites:
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.
Further reading
- Nelson DL, Cox MM (2004). Lehninger Principles of Biochemistry. W. H. Freeman; 4th edition (Hardcover). ISBN 0-7167-4339-
External links