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.

d. Draw the structure of alanine in a solution at pH = 12.

Verified step by step guidance

1. Step 1: Understand the structure of alanine. Alanine is an amino acid with 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 pH of the solution. At pH = 12, the solution is basic, which means it is above the pKa values of both functional groups. This will affect the protonation state of the groups.
3. Step 3: Determine the state of the carboxylic acid group. Since the pH (12) is much higher than the pKa of the carboxylic acid group (2.34), the carboxylic acid group will lose its proton and exist as a carboxylate ion (-COO⁻).
4. Step 4: Determine the state of the amine group. Since the pH (12) is higher than the pKa of the protonated amine group (9.69), the amine group will lose its proton and exist as a neutral amine (-NH2).
5. Step 5: Draw the structure of alanine at pH = 12. The structure will have a carboxylate group (-COO⁻) and a neutral amine group (-NH2), along with the methyl group (-CH3) attached to the alpha carbon.

Key Concepts

Amino Acids

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid contains a central carbon atom bonded to an amino group (–NH2), a carboxylic acid group (–COOH), a hydrogen atom, and a variable side chain (R group) that determines its unique properties. Understanding the structure and function of amino acids is essential for grasping their behavior in different pH environments.

pKa and Ionization

pKa is a measure of the acidity of a proton in a molecule, indicating the pH at which half of the species are deprotonated. In the context of amino acids, the pKa values of the carboxylic acid and the amine groups determine their ionization state at a given pH. At pH 12, which is above the pKa of the carboxylic acid, this group will be deprotonated, while the amine group will remain protonated if its pKa is higher than 12.

Zwitterion Form

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In amino acids, at physiological pH, they often exist as zwitterions, where the amino group is protonated (–NH3+) and the carboxylic acid group is deprotonated (–COO–). At pH 12, alanine will predominantly exist in a form where the carboxylic acid is deprotonated, leading to a net negative charge on the molecule.