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

검증된 단계별 안내

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 = \frac{{pK}_{a1 + {pK}_{a2}}}{2}

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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주요 개념

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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.

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.

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.

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