Phenylalanine is an amino acid found in proteins. In mammals, phenylalanine is an essential amino acid, meaning it must be present in the diet.

Phenylalanine's phenyl side chain classifies it as an aromatic amino acid. The aromatic amino acids, like most compounds carrying conjugated rings, exhibit strong absorption of light in the near-ultraviolet region of the spectrum (Figure 5.6). This absorption is frequently used for the analytical detection of proteins.

Phenylalanine, together with the aliphatic amino acids valine, leucine, and isoleucine, are the most hydrophobic amino acids. The more hydrophobic amino acids are usually found in the interior of a protein molecule, where they are shielded from water.

Phenylalanine is part of the synthetic metabolic pathway in making alkaloids.