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The isoelectric point (pI, pH(I), IEP), is the pH at which a particular molecule carries no net electrical charge. The standard nomenclature to represent the isoelectric point is pH(I),〔Acceptable variants on pH(I) would include pHI, pHIEP, ''etc''; the main point is that one cannot take the 'power' of I, rather one measures the pH subject to a nominated condition.〕 although pI is also commonly seen,〔IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook. Last update: 2014-02-24; version: 2.3.3. DOI of this term: doi:10.1351/goldbook.I03275.〕 and is used in this article for brevity. 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 (H+). 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 by net charge in a polyacrylamide gel using either QPNC-PAGE or 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. == 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 pKas of this molecule.〔For derivation of this expression see acid dissociation constant〕 : The pH of an electrophoretic gel is determined by the 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「isoelectric point」の詳細全文を読む スポンサード リンク
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