Multiple Choice
What is the pI of His?
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3. Amino Acids
Isoelectric Point of Amino Acids with Ionizable R-Groups
Multiple Choice
Calculate the pI of Asp.
A
1.4
B
3
C
5.95
D
6.85
Verified step by step guidance1
Identify the ionizable groups in aspartic acid (Asp). Aspartic acid has three ionizable groups: the α-carboxyl group, the α-amino group, and the side chain carboxyl group.
Determine the pKa values for each ionizable group. From the image, the pKa values are: pKa1 (α-carboxyl group) = 2.1, pKa2 (α-amino group) = 9.8, and pKR (side chain carboxyl group) = 3.9.
Understand 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 Asp, the pI is calculated using the average of the pKa values of the two acidic groups.
Calculate the pI by averaging the pKa values of the two carboxyl groups: pKa1 and pKR. This is because at the pI, the molecule will be neutral, with the carboxyl groups deprotonated and the amino group protonated.
Use the formula: pI = (pKa1 + pKR) / 2. Substitute the known values: pI = (2.1 + 3.9) / 2. This will give you the pI of aspartic acid.
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