Draw the structure of the predominant form of
(a) isoleucine at pH 11.

Verified step by step guidance

1. Step 1: Understand the structure of isoleucine. Isoleucine is an amino acid with the following functional groups: an amino group (-NH₂), a carboxylic acid group (-COOH), and a side chain containing a hydrocarbon group. Its molecular formula is C₆H₁₃NO₂.
2. Step 2: Recall the pH-dependent behavior of amino acids. At pH 11, the environment is basic, meaning the amino acid will lose protons from its acidic functional groups. Specifically, the carboxylic acid group (-COOH) will deprotonate to form a carboxylate ion (-COO⁻), and the amino group (-NH₂) will remain unprotonated as -NH₂.
3. Step 3: Identify the predominant form of isoleucine at pH 11. The carboxylic acid group will exist as -COO⁻, and the amino group will remain as -NH₂. The side chain does not have ionizable groups, so it remains unchanged.
4. Step 4: Draw the structure of isoleucine at pH 11. Represent the backbone with the amino group (-NH₂) on one end, the carboxylate group (-COO⁻) on the other end, and the side chain attached to the central carbon atom. Ensure the stereochemistry of isoleucine is correct, as it is a chiral molecule.
5. Step 5: Verify the structure. Double-check that the functional groups are correctly represented for the basic pH environment and that the stereochemistry of the molecule is accurate.

Key Concepts

Amino Acid Structure

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid has a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group). The structure of isoleucine, an essential amino acid, includes a branched-chain aliphatic side group, which influences its properties and behavior in biological systems.

pH and Ionization

pH is a measure of the acidity or basicity of a solution, affecting the ionization state of functional groups in molecules. At higher pH levels, such as pH 11, amino acids tend to lose protons from their carboxyl groups and may also affect the amino group. Understanding how pH influences the ionization of amino acids is crucial for predicting their predominant forms in different environments.

Zwitterion Formation

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In the case of amino acids, at physiological pH, they often exist as zwitterions, where the amino group is protonated (-NH3+) and the carboxyl group is deprotonated (-COO-). At pH 11, isoleucine will predominantly exist in a deprotonated form, affecting its solubility and reactivity in biological systems.