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

Isoelectric Point

Organic Chemistry

29. Amino Acids

Isoelectric Point

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1:55 minutes

Problem 11d

Textbook Question

d. Which amino acid has a greater negative charge at pH = 6.20, glycine or methionine?

Verified step by step guidance

Determine the pKa values of the functional groups in glycine and methionine. Glycine has a carboxyl group (pKa ≈ 2.34) and an amino group (pKa ≈ 9.60). Methionine also has a carboxyl group (pKa ≈ 2.28) and an amino group (pKa ≈ 9.21). Neither amino acid has an ionizable side chain.

At pH = 6.20, compare the pH to the pKa values of the carboxyl and amino groups for each amino acid. If pH > pKa, the group will be deprotonated; if pH < pKa, the group will be protonated.

For both glycine and methionine, the carboxyl group (pKa ≈ 2.3) will be deprotonated at pH = 6.20, resulting in a negatively charged carboxylate ion (\( \text{COO}^- \)).

For both glycine and methionine, the amino group (pKa ≈ 9.5 for glycine and 9.2 for methionine) will remain protonated at pH = 6.20, resulting in a positively charged ammonium ion (\( \text{NH}_3^+ \)).

Since neither glycine nor methionine has an ionizable side chain, the net charge of each amino acid at pH = 6.20 will depend only on the charges of the carboxyl and amino groups. Both amino acids will have the same net charge at this pH, so neither has a greater negative 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 R group (side chain). The nature of the R group determines the amino acid's properties, including its charge at different pH levels.

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 given pH, the carboxyl group of an amino acid typically donates a proton, becoming negatively charged, while the amino group can accept a proton, becoming positively charged. The overall charge of the amino acid depends on the pH relative to its pKa values.

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. For glycine and methionine, their pI values help determine their charge at pH 6.20. Amino acids with pI values lower than the solution's pH will carry a net negative charge, while those with higher pI values will be positively charged.

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Related Practice

Textbook Question

a. Which amino acid has the lowest pI value?

b. Which amino acid has the highest pI value?

Textbook Question

(a) The isoelectric point (pI) of phenylalanine is pH 5.5. Draw the structure of the major form of phenylalanine at pH values of 1, 5.5, and 11.

Textbook Question

The isoelectric point of glutamic acid is pH 3.2. Draw the structures of the major forms of glutamic acid at pH values of 1, 3.2, 7, and 11. Explain why the side-chain carboxylic acid is a weaker acid than the acid group next to the α-carbon atom.

Textbook Question

Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn–Ingold–Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.

(b) (S)-Alanine is an L-amino acid (Figure 24-2). Is (R)-cysteine a d-amino acid or an L-amino acid?

Textbook Question

Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:

a. aspartate before serine

Textbook Question

Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50:

b. serine before alanine

Textbook Question

Which has a higher percentage of negative charge at physiological pH (7.4), leucine with pI = 5.98 or asparagine with pI = 5.43?

Textbook Question

Draw the electrophoretic separation of Trp, Cys, and His at pH 6.0.

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