Textbook Question
Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at each of the following pH values:
d. pH=12
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29. Amino Acids
Acid-Base Properties of Amino Acids
5:57 minutesProblem 8b
Textbook Question
Draw the predominant form for glutamate in a solution with the following
pH: b. 3
Verified step by step guidance1
Determine the pKa values of the functional groups in glutamate. Glutamate has three ionizable groups: the α-carboxyl group (pKa ≈ 2.1), the α-amino group (pKa ≈ 9.5), and the side-chain carboxyl group (pKa ≈ 4.1).
Compare the pH of the solution (pH = 3) to the pKa of each functional group to determine their protonation states. If pH < pKa, the group will be protonated; if pH > pKa, the group will be deprotonated.
Analyze the α-carboxyl group (pKa ≈ 2.1): Since pH (3) > pKa (2.1), the α-carboxyl group will be deprotonated, forming a carboxylate ion (\( \text{COO}^- \)).
Analyze the side-chain carboxyl group (pKa ≈ 4.1): Since pH (3) < pKa (4.1), the side-chain carboxyl group will remain protonated as \( \text{COOH} \).
Analyze the α-amino group (pKa ≈ 9.5): Since pH (3) < pKa (9.5), the α-amino group will remain protonated as \( \text{NH}_3^+ \). Combine these observations to draw the predominant form of glutamate at pH 3.
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Video duration:
5mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Amino Acid Structure
Amino acids, the building blocks of proteins, consist of a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group. Glutamate, specifically, has a side chain that contains a carboxyl group, making it an acidic amino acid. Understanding the structure of glutamate is essential for predicting its behavior in different pH environments.
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Charged Amino Acids
pH and Protonation States
pH is a measure of the acidity or basicity of a solution, influencing the protonation state of molecules. At low pH (like 3), amino acids can exist in their protonated forms, where the carboxyl group is likely to be protonated (–COOH) and the amino group remains protonated (–NH3+). This affects the overall charge and predominant form of glutamate in solution.
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Zwitterions
Zwitterions are molecules that have both positive and negative charges but are overall neutral. In the context of amino acids, at physiological pH, they often exist as zwitterions. However, at a low pH like 3, glutamate will predominantly exist in a protonated form, which is crucial for understanding its behavior in biochemical processes and interactions.
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