Multiple Choice
Draw Glu and calculate its isoelectric point. pKa1 = 9.67. pKa2 = 2.19. pKR = 4.25.
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3. Amino Acids
Isoelectric Point of Amino Acids with Ionizable R-Groups
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What is the pI of His?
A
7.59
B
6.58
C
3.91
D
5.5
Verified step by step guidance1
Identify the ionizable groups in histidine: the carboxyl group (pKa2 = 1.82), the amino group (pKa1 = 9.17), and the imidazole side chain (pKR = 6.0).
Determine the charge of each group at different pH values: the carboxyl group is negatively charged above pH 1.82, the amino group is positively charged below pH 9.17, and the imidazole group is positively charged below pH 6.0.
To find the isoelectric point (pI), identify the pH at which the molecule has a net charge of zero. This occurs when the positive and negative charges balance each other.
For histidine, the pI is calculated by averaging the pKa values that correspond to the ionization states where the net charge is zero. In this case, average the pKa values of the amino group and the imidazole side chain: (pKa1 + pKR) / 2.
Calculate the average: (9.17 + 6.0) / 2. This will give you the pI of histidine.
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