A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

e. Is there a pH at which alanine is uncharged (that is, neither group has a charge)?
f. At what pH does alanine have no net charge (that is, the amount of negative charge is the same as the amount of positive charge)?

Verified step by step guidance

1. Step 1: Understand the structure of alanine. Alanine is an amino acid with two functional groups: a carboxylic acid group (-COOH) and a protonated amine group (-NH3+). The pKa values provided are 2.34 for the carboxylic acid group and 9.69 for the protonated amine group.
2. Step 2: Analyze the question about the pH at which alanine is uncharged. For alanine to be uncharged, neither the carboxylic acid group nor the amine group can carry a charge. This would require the carboxylic acid group to remain protonated (-COOH) and the amine group to remain neutral (-NH2). However, this situation is not possible under normal physiological pH ranges because the carboxylic acid group will deprotonate at pH > 2.34, and the amine group will protonate at pH < 9.69.
3. Step 3: Address the question about the pH at which alanine has no net charge. Alanine will have no net charge when the positive charge from the protonated amine group (-NH3+) is balanced by the negative charge from the deprotonated carboxylic acid group (-COO−). This occurs at the isoelectric point (pI), which is calculated as the average of the two pKa values.
4. Step 4: Calculate the isoelectric point (pI). The formula for the isoelectric point is: $\mathrm{pI}=\frac{{\mathrm{pK}}_{\mathrm{a1}}+{\mathrm{pK}}_{\mathrm{a2}}}{2}$, where pKa1 is the pKa of the carboxylic acid group (2.34) and pKa2 is the pKa of the protonated amine group (9.69).
5. Step 5: Interpret the result. The calculated pI represents the pH at which alanine exists predominantly in its zwitterionic form, with no net charge. At this pH, the carboxylic acid group is deprotonated (-COO−), and the amine group is protonated (-NH3+), balancing the charges.

Key Concepts

Amino Acid Structure

Amino acids are organic compounds that contain both an amine group (-NH2) and a carboxylic acid group (-COOH). In the case of alanine, it has a side chain that is a methyl group (-CH3). The presence of these functional groups allows amino acids to act as both acids and bases, which is crucial for understanding their behavior in different pH environments.

pKa Values

The pKa value is a measure of the strength of an acid in solution, indicating the pH at which half of the species are deprotonated. For alanine, the pKa of the carboxylic acid group is 2.34, and the pKa of the protonated amine group is 9.69. These values help determine the charge state of alanine at various pH levels, which is essential for answering questions about its net charge.

Zwitterion Formation

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In the case of alanine, it exists as a zwitterion at a specific pH, where the carboxylic acid group is deprotonated (negatively charged) and the amine group is protonated (positively charged). Understanding the pH at which alanine exists as a zwitterion is key to determining when it has no net charge.