(S)-2-Amino-3-(pyridin-4-yl)propanoic acid dihydrochloride, also known as L-4-pyridylalanine dihydrochloride, is a non-proteinogenic amino acid derivative. It features a pyridine ring at the 4-position of the side chain, making it a structural analog of phenylalanine. This compound is typically supplied as a white to off-white crystalline powder and is highly soluble in water. This unnatural amino acid serves as a crucial building block in peptide chemistry and medicinal chemistry research. Its primary application is in the synthesis of peptide-based drugs and bioactive peptides, where it is incorporated to modulate pharmacokinetic properties, enhance receptor binding affinity, or introduce specific chemical functionalities. The pyridyl side chain can participate in hydrogen bonding and π-stacking interactions, making it valuable for studying protein-protein interactions and designing enzyme inhibitors. It is also used in the preparation of targeted protein degraders (PROTACs) and other bifunctional molecules where a specific heterocyclic recognition element is required. As a chiral amino acid derivative, it is essential for constructing stereochemically defined compounds. Handling requires standard precautions for laboratory chemicals, and it should be stored in a cool, dry place.