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.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

Verified step by step guidance

1. Step 1: Understand the structure of alanine. Alanine is an amino acid with a central alpha carbon bonded to a carboxylic acid group (-COOH), an amine group (-NH2), a hydrogen atom, and a methyl group (-CH3). At physiological pH (7.4), the carboxylic acid group and the amine group will exist in their ionized forms.
2. Step 2: Analyze the pKa values provided. The pKa of the carboxylic acid group is 2.32, meaning it will lose a proton and exist as a carboxylate ion (-COO⁻) at pH 7.4, which is above its pKa. The pKa of the amine group is 9.62, meaning it will remain protonated as -NH3⁺ at pH 7.4, which is below its pKa.
3. Step 3: Determine the ionization state of alanine at physiological pH. Since the pH is higher than the pKa of the carboxylic acid group, it will be deprotonated (-COO⁻). Since the pH is lower than the pKa of the amine group, it will remain protonated (-NH3⁺).
4. Step 4: Draw the structure of alanine at physiological pH. The alpha carbon will be bonded to the following groups: a carboxylate ion (-COO⁻), a protonated amine group (-NH3⁺), a hydrogen atom, and a methyl group (-CH3).
5. Step 5: Verify the zwitterionic nature of alanine at physiological pH. Alanine exists as a zwitterion, meaning it has both a positive charge (on the -NH3⁺ group) and a negative charge (on the -COO⁻ group) at pH 7.4.

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). The properties of the side chain determine the characteristics of the amino acid, influencing protein structure and function.

pKa and Acid-Base Chemistry

pKa is a measure of the strength of an acid in solution, indicating the pH at which half of the acid is dissociated. In the context of amino acids, the pKa values of the carboxylic acid and the protonated amine groups help determine their ionization states at a given pH. At physiological pH (7.4), the carboxylic acid group is typically deprotonated, while the amine group remains protonated, influencing the overall charge of the amino acid.

Zwitterion Formation

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In amino acids like alanine at physiological pH, the carboxylic acid group loses a proton (–COO⁻), while the amino group retains a proton (–NH₃⁺). This results in a zwitterionic form, which is crucial for the solubility and reactivity of amino acids in biological systems.