Isoelectric Point
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The isoelectric point (pI), sometimes abbreviated to IEP, is the pH at which a particular molecule or surface carries no net electrical charge.
Amphoteric molecules called zwitterions 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 its 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 layer. 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.
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. Biological amphoteric molecules such as proteins 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 according to their isoelectric point (overall charge) on a polyacrylamide gel using a technique called isoelectric focusing, which uses a pH
gradient to separate proteins. Isoelectric focusing is also the first step in 2-D gel polyacrylamide gel electrophoresis.
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As you know,Amino acids are less soluble in water at isoelectric point and will precipitate out.I dissolved 10 gram of alanine in 200 ml of water
and reached PH to isoelectric point of alanine( i tried 6.0 and 6.11) and then i put it in cool place(4-5c) for 10 hours and got nothing!No
precipitate
Also i did this experiment with glycine and again got nothing!
somebody know what is problem?i should use less solvent?
How much are amino acids soluble in water at isoelectric point?
[Edited on 6-12-2012 by Waffles SS]
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