Draw the structure of the predominant form of
(e) a mixture of alanine, lysine, and aspartic acid at (iii) pH 2.

Verified step by step guidance

1. Identify the pKa values for the ionizable groups in alanine, lysine, and aspartic acid.
2. At pH 2, compare the pH to the pKa values to determine the protonation state of each functional group.
3. For alanine, consider the carboxyl group (pKa ~2) and the amino group (pKa ~9.7). At pH 2, the carboxyl group is mostly protonated (COOH) and the amino group is protonated (NH3^+).
4. For lysine, consider the carboxyl group (pKa ~2.2), the amino group (pKa ~9), and the side chain amino group (pKa ~10.5). At pH 2, both the carboxyl group and the amino groups are protonated.
5. For aspartic acid, consider the carboxyl group (pKa ~2), the amino group (pKa ~9.8), and the side chain carboxyl group (pKa ~3.9). At pH 2, both carboxyl groups are protonated, and the amino group is protonated.

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 R group that determines its unique properties. Understanding the structure of amino acids is essential for predicting their behavior in different pH environments.

Zwitterion Formation

At certain pH levels, amino acids can exist as zwitterions, which are molecules that have both positive and negative charges but are overall neutral. This occurs when the amino group is protonated (-NH3+) and the carboxyl group is deprotonated (-COO-). The formation of zwitterions is crucial for understanding the predominant forms of amino acids in solution, especially at varying pH levels.

pH and Amino Acid Ionization

The pH of a solution affects the ionization state of amino acids. At low pH (like pH 2), amino acids tend to be protonated, leading to a higher concentration of positively charged species. This concept is vital for predicting the predominant forms of alanine, lysine, and aspartic acid in acidic conditions, as their side chains will also influence their overall charge and structure.