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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2:41 minutes

Problem 7

Textbook Question

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

Verified step by step guidance

Determine the pKa values of the functional groups in each amino acid (Ala, Lys, and Asp). For Ala, consider the amino group (pKa ~9.7) and carboxyl group (pKa ~2.3). For Lys, include the amino group (pKa ~9.7), carboxyl group (pKa ~2.2), and side-chain amino group (pKa ~10.5). For Asp, include the amino group (pKa ~9.7), carboxyl group (pKa ~2.1), and side-chain carboxyl group (pKa ~3.9).

Evaluate the net charge of each amino acid at pH 9.7. Compare the pH to the pKa values of each functional group to determine whether each group is protonated or deprotonated. For example, if pH > pKa, the group is deprotonated; if pH < pKa, the group is protonated.

Assign the overall charge to each amino acid based on the protonation states of its functional groups. For instance, at pH 9.7, the amino group of Ala will be neutral, and the carboxyl group will be negatively charged, resulting in a net charge of -1. Repeat this process for Lys and Asp.

Predict the direction of migration for each amino acid during electrophoresis at pH 9.7. Amino acids with a net negative charge will migrate toward the anode (positive electrode), while those with a net positive charge will migrate toward the cathode (negative electrode). Neutral amino acids will not migrate significantly.

Draw the electrophoretic separation by placing Ala, Lys, and Asp on a diagram. Indicate their starting positions and the direction of their migration based on their net charges. Label the anode and cathode to show the orientation of the electric field.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Isoelectric Point (pI)

The isoelectric point (pI) is the pH at which a molecule, such as an amino acid, carries no net electrical charge. For amino acids, this is crucial because it determines their solubility and behavior in an electric field. At pH values below their pI, amino acids are positively charged, while at pH values above their pI, they are negatively charged.

Electrophoresis

Electrophoresis is a technique used to separate charged particles, such as amino acids, based on their size and charge under an electric field. When an electric current is applied, molecules migrate towards the electrode of opposite charge. The rate of migration depends on the charge-to-mass ratio of the molecules, allowing for their separation in a gel or solution.

Amino Acid Properties

Amino acids have unique side chains that influence their charge at different pH levels. For instance, at pH 9.7, the amino acid alanine (Ala) is neutral, lysine (Lys) is positively charged due to its basic side chain, and aspartic acid (Asp) is negatively charged because of its acidic side chain. Understanding these properties is essential for predicting their behavior during electrophoresis.

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Draw the structure of the predominant form of

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

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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

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).

Multiple Choice

Why do amino acids exist in the Zwitterionic form at physiological pH?

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