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 4e(iii)

Textbook Question

Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (iii) pH 2.

Verified step by step guidance

Step 1: Understand the problem. At pH 2, the environment is highly acidic. This means that the amino acids will predominantly exist in their protonated forms. Amino acids have functional groups that can gain or lose protons depending on the pH, such as the amino group (-NH₂), carboxylic acid group (-COOH), and side chain groups.

Step 2: Analyze the structure of alanine. Alanine has a simple side chain (-CH₃). At pH 2, the amino group (-NH₂) will be protonated to form -NH₃⁺, and the carboxylic acid group (-COOH) will remain protonated.

Step 3: Analyze the structure of lysine. Lysine has a side chain with an amino group (-NH₂). At pH 2, the amino group in the side chain will also be protonated to form -NH₃⁺, along with the main amino group (-NH₃⁺). The carboxylic acid group (-COOH) will remain protonated.

Step 4: Analyze the structure of aspartic acid. Aspartic acid has a side chain with a carboxylic acid group (-COOH). At pH 2, both the main carboxylic acid group and the side chain carboxylic acid group will remain protonated, and the amino group (-NH₂) will be protonated to form -NH₃⁺.

Step 5: Combine the information. Draw the structures of alanine, lysine, and aspartic acid with their respective protonated forms at pH 2. Ensure that all amino groups are protonated (-NH₃⁺) and all carboxylic acid groups are in their protonated form (-COOH). This represents the predominant form of the mixture at pH 2.

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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 low pH (like pH 2), amino acids tend to be protonated, meaning the carboxyl group (-COOH) remains protonated, while the amino group (-NH2) is also protonated to form -NH3+. This affects the overall charge and structure of the amino acids in solution.

Zwitterions

Zwitterions are molecules that have both positive and negative charges but are overall neutral. At physiological pH, many amino acids exist as zwitterions, where the amino group is positively charged and the carboxyl group is negatively charged. Understanding zwitterionic forms is crucial for predicting the predominant species of amino acids in different pH conditions, such as the acidic environment at pH 2.

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Draw the structure of the predominant form of

(b) proline at pH 2.

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Draw the structure of the predominant form of

(e) a mixture of alanine, lysine, and aspartic acid at (i) pH 6;

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Draw the structure of the predominant form of

(a) isoleucine at pH 11.

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Draw the structure of the predominant form of

(d) glutamic acid at pH 7.

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Although tryptophan contains a heterocyclic amine, it is considered a neutral amino acid. Explain why the indole nitrogen of tryptophan is more weakly basic than one of the imidazole nitrogens of histidine.

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Aspartame (Nutrasweet®) is a remarkably sweet-tasting dipeptide ester. Complete hydrolysis of aspartame gives phenyl alanine, aspartic acid, and methanol. Mild incubation with carboxypeptidase has no effect on aspartame. Treatment of aspartame with phenyl isothiocyanate, followed by mild hydrolysis, gives the phenylthiohydantoin of aspartic acid. Propose a structure for aspartame.

Textbook Question

Draw the following amino acids in all possible protonation states as you move from high pH to low pH.

b. His

Textbook Question

Draw the electrophoretic separation of Ala, Lys, and Asp at pH 9.7.

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