(a) The isoelectric point (pI) of phenylalanine is pH 5.5. Draw the structure of the major form of phenylalanine at pH values of 1, 5.5, and 11.

Verified step by step guidance

1. Identify the functional groups in phenylalanine: an amino group (-NH2), a carboxyl group (-COOH), and a phenyl group (aromatic ring).
2. At pH 1, the environment is highly acidic. The amino group will be protonated to form -NH3^+, and the carboxyl group will remain as -COOH.
3. At the isoelectric point (pH 5.5), phenylalanine exists as a zwitterion. The amino group is protonated (-NH3^+) and the carboxyl group is deprotonated (-COO^-).
4. At pH 11, the environment is basic. The amino group will lose a proton to become -NH2, and the carboxyl group will remain deprotonated as -COO^-.
5. Draw the structures for each pH condition, showing the protonation states of the amino and carboxyl groups.

Key Concepts

Isoelectric Point (pI)

The isoelectric point (pI) is the pH at which a molecule, such as an amino acid, carries no net electrical charge. For phenylalanine, this occurs at pH 5.5, meaning that at this pH, the positive and negative charges on the molecule balance each other out. Understanding pI is crucial for predicting the behavior of amino acids in different pH environments.

Amino Acid Structure

Amino acids have a general structure consisting of a central carbon atom, an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group). For phenylalanine, the side chain is a benzyl group, which influences its properties and behavior in solution. Recognizing how these groups interact with pH is essential for drawing the correct structures at different pH levels.

Protonation and Deprotonation

Protonation and deprotonation refer to the addition or removal of protons (H+) from a molecule, affecting its charge and structure. At low pH (1), amino acids are typically protonated, leading to a positive charge. At high pH (11), they are deprotonated, resulting in a negative charge. Understanding these processes is vital for determining the predominant form of phenylalanine at various pH levels.