Multiple Choice
Calculate the pI of Arg. pKa1 = 9. pKa2 = 2.2. pKR = 12.5.
3. Amino Acids
Isoelectric Point of Amino Acids with Ionizable R-Groups
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Join thousands of students who trust us to help them ace their exams!Watch the first videoMultiple Choice
Draw Glu and calculate its isoelectric point. pKa1 = 9.67. pKa2 = 2.19. pKR = 4.25.
A
7.48
B
3.22
C
6.96
D
5.93
Verified step by step guidance1
Identify the structure of Glutamic acid (Glu), which is an amino acid with the following groups: an amino group (-NH2), a carboxyl group (-COOH), a side chain carboxyl group (-COOH), and a hydrogen atom attached to the alpha carbon.
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 Glu, the pI is calculated using the pKa values of the carboxyl groups and the side chain.
List the pKa values: pKa1 for the alpha carboxyl group is 2.19, pKa2 for the alpha amino group is 9.67, and pKR for the side chain carboxyl group is 4.25.
To find the isoelectric point (pI) for Glu, average the pKa values of the two acidic groups that lose protons first. These are pKa1 (2.19) and pKR (4.25).
Calculate the isoelectric point (pI) using the formula: pI = (pKa1 + pKR) / 2. This will give you the pH at which Glu has no net charge.
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