Isoelectronic point

Isoelectronic point - The other two cases introduce other...

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Isoelectronic point, pI * The isoelectronic point or isoionic point is the pH at which the amino acid does not migrate in an electric field. * This means it is the pH at which the amino acid is neutral, i.e. the zwitterion form is dominant. * The pI is given by the average of the pKas that involve the zwitterion, i.e. that give the boundaries to its existence. There are 3 cases to consider. ... * neutral side chains These amino acids are characterised by two pKas : pKa1 and pKa2 for the carboxylic acid and the amine respectively. The isoelectronic point will be halfway between, or the average of, these two pKas, i.e. pI = 1/2 (pKa1 + pKa2). This is most readily appreciated when you realise that at very acidic pH (below pKa1) the amino acid will have an overall +ve charge and at very basic pH (above pKa2 ) the amino acid will have an overall -ve charge. For the simplest amino acid, glycine, pKa1= 2.34 and pKa2 = 9.6, pI = 5.97.
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Unformatted text preview: The other two cases introduce other ionisable groups in the side chain "R" described by a third acid dissociation constant, pKaR * acidic side chains The pI will be at a lower pH because the acidic side chain introduces an "extra" negative charge. So the neutral form exists under more acidic conditions when the extra -ve has been neutralised. For example, for aspartic acid shown below, the neutral form is dominant between pH 1.88 and 3.65, pI is halfway between these two values, i.e. pI = 1/2 (pKa1 + pKa3), so pI = 2.77. * basic side chains The pI will be at a higher pH because the basic side chain introduces an "extra" positive charge. So the neutral form exists under more basic conditions when the extra +ve has been neutralised. For example, for histidine, , the neutral form is dominant between pH 6.00 and 9.17, pI is halfway between these two values, i.e. pI = 1/2 (pKa2 + pKa3), so pI = 7.59....
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Isoelectronic point - The other two cases introduce other...

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