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The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). [1] However, pI is also used. [2] For brevity, this article uses pI.
[72] [73] [74] The isoelectric point of a given molecule is a function of its pK values, so different molecules have different isoelectric points. This permits a technique called isoelectric focusing , [ 75 ] which is used for separation of proteins by 2-D gel polyacrylamide gel electrophoresis .
In approximately neutral aqueous solution (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is the zwitterion H 3 N + −RCH−COO −. The pH at which the average charge is zero is known as the molecule's isoelectric point.
From the titration of protonatable group, one can read the so-called pK a 1 ⁄ 2 which is equal to the pH value where the group is half-protonated (i.e. when 50% such groups would be protonated). The pK a 1 ⁄ 2 is equal to the Henderson–Hasselbalch pK a (pK HH a) if the titration curve follows the Henderson–Hasselbalch equation. [14]
The iso-electric point is the pH value at which the zeta potential is approximately zero. At a pH near the iso-electric point (± 2 pH units), colloids are usually unstable; the particles tend to coagulate or flocculate. Such titrations use acids or bases as titration reagents. Tables of iso-electric points for different materials are available ...
Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isoelectric point, and some amino acids (in particular, with nonpolar side chains) can be isolated by precipitation from ...
The two dimensions that proteins are separated into using this technique can be isoelectric point, protein complex mass in the native state, or protein mass. [citation needed] The separation by isoelectric point is called isoelectric focusing. Thereby, a pH gradient is applied to a gel and an electric potential is applied across the gel, making ...
Isoelectric focusing (IEF), also known as electrofocusing, is a technique for separating different molecules by differences in their isoelectric point (pI). [ 1 ] [ 2 ] It is a type of zone electrophoresis usually performed on proteins in a gel that takes advantage of the fact that overall charge on the molecule of interest is a function of the ...