Phenylalanine is an essential amino acid that is also one of the aromatic amino acids that exhibit ultraviolet radiation absorption properties with a large extinction coefficient. This characteristic is often used as an analytical tool to quantify the amount of protein in a sample. Phenylalanine plays a key role in the biosynthesis of other amino acids and some neurotransmitters. It is the most commonly found aromatic amino acid in proteins and enzymes with a molar ratio of 3.5 percent compared to the other amino acids, about double the amount of any other aromatic amino acid.