Practice: Calculate the pI value for histidine. pKa1 (carboxyl group) = 1.60, pKa2 (ammonium group) = 9.28, pKa3 (R-group) = 5.97.
Isoelectric and Isoionic PointsÂ
Concept #1: Isoelectric or isoionic points represent the pH where a polyprotic acid doesn’t migrate to an electric field because it’s neutral.
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Concept #2: Isoelectric and Isoionic points allow us to determine the pH of diprotic and polyprotic systems.
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Example #1: Calculate the isoelectric and isoionic pH of 0.025 M glutamine. pKa1 = 2.19, pKa2 = 9.00.
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Example #2: Draw the structures and charge of aspartic acid at pH = 9.82.
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Practice: Calculate the pI value for gluatamic acid. pKa1 (carboxyl group) = 2.16, pKa2 (ammonium group) = 9.96, pKa3 (R-group) = 4.30.
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