Multiple Choice
Calculate the pI value for gluatamic acid. pKa1 (carboxyl group) = 2.16, pKa2 (ammonium group) = 9.96, pKa3 (R-group) = 4.30.
9. Polyprotic Acid-Base Equilibria
Isoelectric and Isoionic pH
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Calculate the pI value for histidine. pKa1 (carboxyl group) = 1.60, pKa2 (ammonium group) = 9.28, pKa3 (R-group) = 5.97.
A
7.63
B
1.65
C
5.44
D
3.74
E
5.62
Verified step by step guidance1
Identify the ionizable groups in histidine: the carboxyl group, the ammonium group, and the R-group (imidazole side chain).
Determine the pKa values for each ionizable group: pKa1 = 1.60 (carboxyl group), pKa2 = 9.28 (ammonium group), and pKa3 = 5.97 (R-group).
Recognize that the isoelectric point (pI) is the pH at which the molecule carries no net electric charge. For amino acids with three ionizable groups, the pI is calculated using the average of the pKa values that surround the zwitterionic form.
For histidine, the zwitterionic form is between the R-group and the ammonium group. Therefore, use the pKa values of the R-group and the ammonium group: pKa3 = 5.97 and pKa2 = 9.28.
Calculate the pI by averaging the two pKa values: pI = (pKa3 + pKa2) / 2.
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