Multiple Choice
Calculate the pI for arginine (the pKa values—or the pKa values for the conjugate acids of bases—are given).
29. Amino Acids
Acid-Base Properties of Amino Acids
6:06 minutesProblem 63d
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
Verified step by step guidance1
Step 1: Identify the ionizable groups in the hexapeptide Lys-Ser-Asp-Cys-His-Tyr. These include the N-terminal amino group, the C-terminal carboxyl group, and the side chains of Lys (lysine), Asp (aspartic acid), Cys (cysteine), His (histidine), and Tyr (tyrosine).
Step 2: Determine the pKa values of the ionizable groups. Approximate pKa values are: N-terminal amino group (~9), C-terminal carboxyl group (~2), Lys side chain (~10.5), Asp side chain (~3.9), Cys side chain (~8.3), His side chain (~6), and Tyr side chain (~10).
Step 3: Compare the pH (12) to the pKa of each ionizable group to determine its protonation state. At pH > pKa, the group will be deprotonated, and at pH < pKa, the group will be protonated. For example, the Lys side chain (pKa ~10.5) will be deprotonated and neutral at pH 12, while the Asp side chain (pKa ~3.9) will be deprotonated and negatively charged.
Step 4: Assign the charge to each ionizable group based on its protonation state. For instance, the N-terminal amino group will be deprotonated and neutral, the C-terminal carboxyl group will be deprotonated and negatively charged, and so on for the side chains of Lys, Ser, Asp, Cys, His, and Tyr.
Step 5: Sum up the charges of all ionizable groups to determine the overall charge of the hexapeptide at pH 12. Ensure that you account for each group’s individual charge contribution to the total charge.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Amino Acid Structure and Properties
Amino acids, the building blocks of peptides, have a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group. The properties of the R group determine the amino acid's characteristics, including its charge at different pH levels. Understanding the structure and properties of the amino acids in the hexapeptide is essential for predicting their behavior in solution.
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pH and Charge States
pH is a measure of the acidity or basicity of a solution, which affects the ionization of functional groups in amino acids. At high pH values, such as pH 12, amino groups tend to lose protons and become neutral, while carboxyl groups are deprotonated and carry a negative charge. This shift in charge states is crucial for determining the overall charge of the hexapeptide at a given pH.
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Peptide Charge Calculation
The overall charge of a peptide is determined by the sum of the charges of its constituent amino acids at a specific pH. Each amino acid in the hexapeptide Lys-Ser-Asp-Cys-His-Tyr will contribute to the total charge based on its ionization state at pH 12. By analyzing the charge of each amino acid, one can accurately determine the net charge of the hexapeptide in that environment.
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