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Ch.18 Amino Acids and Proteins
McMurry - Fundamentals of General, Organic, and Biological Chemistry 8th Edition
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.18 Amino Acids and ProteinsProblem 55
Chapter 18, Problem 55

Which form of aspartic acid in Problem 18.54 is the zwitterion? What is the pI for the zwitterion?
a.
b.
c.

Verified step by step guidance
1
Identify the structure of aspartic acid. Aspartic acid is an amino acid with the general structure: an amino group (-NH₂), a carboxylic acid group (-COOH), a side chain containing a carboxylic acid group (-CH₂COOH), and a central alpha carbon.
Understand the concept of a zwitterion. A zwitterion is a molecule that has both a positive and a negative charge but is overall electrically neutral. For amino acids, this occurs when the amino group is protonated (-NH₃⁺) and one of the carboxylic acid groups is deprotonated (-COO⁻).
Determine the pH range where the zwitterion form of aspartic acid exists. The zwitterion form is predominant at the isoelectric point (pI), which is the pH at which the molecule has no net charge. For aspartic acid, this occurs between the pKa values of the carboxylic acid groups and the amino group.
Calculate the pI for aspartic acid. The pI is the average of the two pKa values that correspond to the ionization states where the zwitterion is formed. For aspartic acid, these are the pKa of the carboxylic acid side chain and the pKa of the alpha carboxylic acid group. Use the formula: pI = pKa1 + pKa22
Substitute the pKa values for aspartic acid into the formula. The typical pKa values for aspartic acid are approximately 2.1 for the alpha carboxylic acid group, 3.9 for the side chain carboxylic acid group, and 9.8 for the amino group. Use the two relevant pKa values (2.1 and 3.9) to calculate the pI.

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Key Concepts

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

Zwitterion

A zwitterion is a molecule that has both positive and negative charges but is overall electrically neutral. In the context of amino acids like aspartic acid, the zwitterionic form typically occurs at a specific pH where the amino group is protonated (positively charged) and the carboxyl group is deprotonated (negatively charged). This dual charge allows zwitterions to play a crucial role in biological systems and influence solubility and reactivity.
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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 this point, the concentrations of the positively and negatively charged forms of the molecule are equal. For aspartic acid, the pI can be calculated using the pKa values of its ionizable groups, which helps determine the pH at which the zwitterionic form predominates.
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Aspartic Acid Structure

Aspartic acid is a polar, negatively charged amino acid with two carboxyl groups and one amino group. Its structure includes a central carbon atom bonded to an amino group (–NH2), a carboxyl group (–COOH), and a side chain that contains another carboxyl group. Understanding its structure is essential for identifying the zwitterionic form and calculating the pI, as the ionization states of these functional groups change with pH.
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Related Practice
Textbook Question

How could you make the zwitterion of aspartic acid more soluble in water?

Textbook Question

Which of the following forms of aspartic acid would you expect to predominate at low pH, neutral pH, and high pH?

a.

b.

c.

Textbook Question

Use the three-letter abbreviations to name all tripeptides that contain valine, methionine, and leucine.

Textbook Question

Proteins are usually least soluble in water at their isoelectric points. Explain.

Textbook Question

 Is phenylalanine hydrophilic or hydrophobic? Explain why.

Textbook Question

At neutral pH, which of the following amino acids has a net positive charge, which has a net negative charge, and which is neutral? (Hint: Draw the various charged forms of each amino acid before deciding.)

a. Asparagine

b. Lysine

c. Proline