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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7:15 minutes

Problem 4e(i)

Textbook Question

Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (i) pH 6;

Verified step by step guidance

Step 1: Understand the problem. You are tasked with determining the predominant form of alanine, lysine, and aspartic acid at pH 6. This involves analyzing the ionization states of their functional groups (amino, carboxyl, and side chains) based on their pKa values.

Step 2: Recall the pKa values of the relevant functional groups. For alanine, the amino group has a pKa of approximately 9.7, and the carboxyl group has a pKa of approximately 2.3. For lysine, the amino group has a pKa of approximately 9.0, the carboxyl group has a pKa of approximately 2.2, and the side chain amino group has a pKa of approximately 10.5. For aspartic acid, the amino group has a pKa of approximately 9.8, the carboxyl group has a pKa of approximately 2.1, and the side chain carboxyl group has a pKa of approximately 3.9.

Step 3: Determine the ionization state of each functional group at pH 6. At this pH, groups with pKa values below 6 will be deprotonated (negative charge for carboxyl groups), and groups with pKa values above 6 will remain protonated (positive charge for amino groups). For alanine, the amino group will be protonated (+1), and the carboxyl group will be deprotonated (-1). For lysine, the amino group will be protonated (+1), the carboxyl group will be deprotonated (-1), and the side chain amino group will remain protonated (+1). For aspartic acid, the amino group will be protonated (+1), the carboxyl group will be deprotonated (-1), and the side chain carboxyl group will be deprotonated (-1).

Step 4: Combine the charges to determine the net charge of each amino acid at pH 6. Alanine will have a net charge of 0 (+1 from the amino group and -1 from the carboxyl group). Lysine will have a net charge of +1 (+1 from the amino group, -1 from the carboxyl group, and +1 from the side chain amino group). Aspartic acid will have a net charge of -1 (+1 from the amino group, -1 from the carboxyl group, and -1 from the side chain carboxyl group).

Step 5: Draw the structures of alanine, lysine, and aspartic acid at pH 6, showing the ionization states of their functional groups. Ensure that the amino groups are protonated where appropriate, the carboxyl groups are deprotonated, and the side chains reflect their correct ionization states. Label each structure clearly to indicate the amino acid and its net 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

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid has a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group) that determines its unique properties. Understanding the structure of amino acids is essential for predicting their behavior in different pH environments.

pH and Ionization

pH is a measure of the acidity or basicity of a solution, influencing the ionization state of amino acids. At a specific pH, amino acids can exist in different forms: cationic, zwitterionic, or anionic. At pH 6, which is near the physiological pH, amino acids like alanine, lysine, and aspartic acid will have distinct charges that affect their overall structure and interactions.

Zwitterion Formation

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. At physiological pH, amino acids typically exist as zwitterions, where the amino group is protonated (-NH3+) and the carboxyl group is deprotonated (-COO-). This form is crucial for understanding the solubility and reactivity of amino acids in biological systems.

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