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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1:28 minutes

Problem 52

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?

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Determine the relationship between the pI (isoelectric point) of each amino acid and the physiological pH (7.4). The pI is the pH at which the amino acid has no net charge. If the pH is higher than the pI, the amino acid will carry a net negative charge due to deprotonation of acidic groups.

For leucine (pI = 5.98), compare the physiological pH (7.4) to its pI. Since 7.4 > 5.98, leucine will exist predominantly in a negatively charged form at physiological pH.

For asparagine (pI = 5.43), compare the physiological pH (7.4) to its pI. Since 7.4 > 5.43, asparagine will also exist predominantly in a negatively charged form at physiological pH.

To determine which amino acid has a higher percentage of negative charge, consider the difference between the physiological pH and the pI for each amino acid. A larger difference indicates a greater degree of deprotonation and thus a higher percentage of negative charge.

Compare the differences: For leucine, the difference is 7.4 - 5.98 = 1.42. For asparagine, the difference is 7.4 - 5.43 = 1.97. Since asparagine has a larger difference, it will have a higher percentage of negative charge at physiological pH.

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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. At pH values below the pI, the molecule tends to be positively charged, while at pH values above the pI, it tends to be negatively charged. Understanding pI is crucial for predicting the charge state of amino acids at physiological pH.

Amino Acid Charge States

Amino acids can exist in different charge states depending on the pH of their environment. At physiological pH (7.4), amino acids with a pI lower than this value will predominantly carry a negative charge, while those with a pI higher will carry a positive charge. This concept is essential for determining the overall charge of amino acids in biological systems.

Physiological pH

Physiological pH refers to the typical pH of human blood and tissues, which is approximately 7.4. At this pH, the ionization states of amino acids and other biomolecules can significantly influence their behavior, interactions, and functions in biological processes. Recognizing the implications of physiological pH is vital for understanding biochemical reactions and molecular interactions.

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Related Practice

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

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

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

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

Open Question

Calculate the isoelectric point of tyrosine (Y)

Open Question

Calculate the isoelectric point of glutamic acid (E)

Textbook Question

Explain why the pI of lysine is the average of the pKa values of its two protonated amino groups.

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