Which of the following amino acids has a polar but non-aromatic side chain?

Threonine is an amino acid that possesses a polar but non-aromatic side chain, making it hydrophilic and capable of forming hydrogen bonds with water and other molecules. Its side chain contains a hydroxyl group (-OH), which contributes to its polar characteristics and allows it to engage in various biochemical interactions, particularly in protein structures and enzymatic functions.

Understanding the nature of threonine's side chain is crucial in the context of protein folding and stability, as polar amino acids often reside on the surface of proteins where they can interact with the aqueous environment, contributing to solubility and reactivity.

In contrast, the other amino acids listed have different side chain characteristics. Tryptophan and phenylalanine have aromatic side chains, which are largely hydrophobic due to the presence of hydrophobic benzene rings. While lysine does have a polar side chain, it is also classified as basic due to the presence of an amine group that can accept protons. Therefore, it is important to note that while polar characteristics are present in lysine, its classification differs from that of threonine which is a non-aromatic polar amino acid.