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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Problem 11c

Textbook Question

c. Which amino acid has the greatest amount of negative charge at pH = 6.20?

Verified step by step guidance

Identify the amino acids that have side chains capable of carrying a negative charge at physiological pH. These are typically the acidic amino acids, such as aspartic acid (Asp, D) and glutamic acid (Glu, E), which have carboxylic acid (-COOH) groups in their side chains.

Determine the pKa values of the side chain carboxylic acid groups for aspartic acid and glutamic acid. The pKa values are approximately 3.9 for aspartic acid and 4.2 for glutamic acid.

Compare the given pH (6.20) to the pKa values of the side chains. At a pH higher than the pKa, the carboxylic acid groups will be deprotonated, resulting in a negative charge. Use the Henderson-Hasselbalch equation to confirm the degree of deprotonation: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \).

Since the pH (6.20) is significantly higher than both pKa values (3.9 and 4.2), the side chains of both aspartic acid and glutamic acid will be almost fully deprotonated and negatively charged. However, consider the total number of carboxylic acid groups in each amino acid to determine which has the greatest overall negative charge.

Conclude that the amino acid with the greatest amount of negative charge at pH 6.20 is the one with the most deprotonated carboxylic acid groups, taking into account both the side chain and the carboxyl group in the backbone.

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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). The properties of the side chain determine the amino acid's characteristics, including its charge at a given pH.

pH and Charge

pH is a measure of the acidity or basicity of a solution, influencing the ionization of functional groups in amino acids. At a specific pH, the carboxyl group of an amino acid typically donates a proton, becoming negatively charged (–COO⁻), while the amino group can accept a proton, becoming positively charged (–NH3⁺). The overall charge of the amino acid depends on the pKa values of these groups relative to the pH.

Isoelectric Point (pI)

The isoelectric point (pI) is the pH at which an amino acid has no net charge, meaning the positive and negative charges balance each other. Amino acids with acidic side chains, such as aspartic acid and glutamic acid, tend to have lower pI values and can carry a negative charge at physiological pH levels. Understanding the pI helps predict the charge of amino acids at specific pH values.

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