At pH 11, what is the most likely ionization state of alanine?

• A. The amino group is protonated and the carboxyl group is deprotonated, resulting in a zwitterion.
• B. Both the amino and carboxyl groups are deprotonated, resulting in a net negative charge.
• C. The amino group is deprotonated and the carboxyl group is protonated, resulting in a neutral molecule.
• D. Both the amino and carboxyl groups are protonated, resulting in a net positive charge.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the structure of alanine. Alanine is an amino acid with an amino group (-NH2), a carboxyl group (-COOH), and a side chain consisting of a methyl group (-CH3). The ionization state of these functional groups depends on the pH of the environment.
2. Step 2: Recall the pKa values of the functional groups in alanine. The amino group typically has a pKa around 9.5, and the carboxyl group has a pKa around 2.1. At a pH higher than the pKa of a group, the group tends to lose a proton (deprotonated state). At a pH lower than the pKa, the group tends to retain a proton (protonated state).
3. Step 3: Analyze the pH given in the problem. The pH is 11, which is higher than the pKa of both the amino group (9.5) and the carboxyl group (2.1). This means both groups are likely to be deprotonated at this pH.
4. Step 4: Determine the ionization state of alanine at pH 11. The amino group (-NH2) will lose a proton and become -NH (neutral), while the carboxyl group (-COOH) will lose a proton and become -COO⁻ (negatively charged). This results in a net negative charge for the molecule.
5. Step 5: Conclude the most likely ionization state. At pH 11, alanine will have both the amino and carboxyl groups deprotonated, resulting in a net negative charge for the molecule.