Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

29. Amino Acids

Acid-Base Properties of Amino Acids

Organic Chemistry

29. Amino Acids

Acid-Base Properties of Amino Acids

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6:06 minutes

Problem 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 guidance

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.

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.

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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