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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3:59 minutes

Problem 46b

Textbook Question

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

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Identify the functional groups in histidine: an amino group (-NH2), a carboxyl group (-COOH), and an imidazole side chain.

At high pH, all groups are deprotonated. Draw histidine with the amino group as -NH2, the carboxyl group as -COO^-, and the imidazole ring with a neutral nitrogen.

As the pH decreases, the carboxyl group will protonate first. Draw histidine with the carboxyl group as -COOH, while the amino group remains -NH2 and the imidazole ring remains neutral.

At a lower pH, the imidazole ring will protonate. Draw histidine with the imidazole ring having a positively charged nitrogen, while the carboxyl group is -COOH and the amino group is still -NH2.

At the lowest pH, the amino group will protonate. Draw histidine with the amino group as -NH3⁺, the carboxyl group as -COOH, and the imidazole ring with a positively charged nitrogen.

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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 composed of an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain (R group) attached to a central carbon atom. Histidine, specifically, has an imidazole ring in its side chain, which plays a crucial role in its behavior under varying pH conditions.

Protonation States

Protonation states refer to the different forms an amino acid can take depending on the pH of the environment. As pH decreases, amino acids gain protons, affecting their charge and structure. Histidine can exist in multiple protonation states due to its imidazole ring, which can accept or donate protons.

pH and Amino Acid Ionization

The pH of a solution influences the ionization of amino acids, affecting their charge and solubility. At high pH, amino acids tend to lose protons, becoming negatively charged, while at low pH, they gain protons, becoming positively charged. Understanding the pKa values of functional groups in histidine is essential for predicting its behavior across different pH levels.

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Textbook Question

Draw the structure of the predominant form of

(d) glutamic acid at pH 7.

Textbook Question

Draw the structure of the predominant form of

(e) a mixture of alanine, lysine, and aspartic acid at (iii) pH 2.

Textbook Question

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

Open Question

Predict the predominant form and net charge of tyrosine (Y) at pH 10. What is the net charge?

Open Question

Determine the net charge of the dipeptide R-C at pH 4.3. (Hint:Peptide bonds do not count)

Multiple Choice

Calculate the pI for arginine (the pK_a values—or the pK_a values for the conjugate acids of bases—are given).

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

Key Concepts

Amino Acid Structure

Protonation States

pH and Amino Acid Ionization

Watch next

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