Multiple Choice
Calculate the pI value for histidine. pKa1 (carboxyl group) = 1.60, pKa2 (ammonium group) = 9.28, pKa3 (R-group) = 5.97.
9. Polyprotic Acid-Base Equilibria
Isoelectric and Isoionic pH
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Calculate the pI value for gluatamic acid. pKa1 (carboxyl group) = 2.16, pKa2 (ammonium group) = 9.96, pKa3 (R-group) = 4.30.
A
6.06
B
7.13
C
3.23
D
5.47
E
7.00
Verified step by step guidance1
Identify the ionizable groups in glutamic acid: the carboxyl group, the ammonium group, and the R-group (side chain carboxyl group).
Recognize that the isoelectric point (pI) is the pH at which the molecule carries no net electric charge. For amino acids with acidic side chains like glutamic acid, the pI is calculated using the pKa values of the two acidic groups.
For glutamic acid, the relevant pKa values are pKa1 (carboxyl group) = 2.16 and pKa3 (R-group) = 4.30, as these are the two acidic groups.
Calculate the pI by taking the average of the two pKa values of the acidic groups: \( pI = \frac{pKa1 + pKa3}{2} \).
Substitute the given pKa values into the formula: \( pI = \frac{2.16 + 4.30}{2} \).
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