Phenylalanine. Molecular model of the amino acid phenylalanine. Its chemical formula is C9.H11.N.O2. Atoms are represented as balls and are colour-coded: carbon (blue), hydrogen (gold), oxygen (red) and nitrogen (dark blue). Amino acids are the building blocks of proteins. Phenylalanine is an essential amino acid. It cannot be synthesised by the body and so needs to be obtained from food. It is found in most protein- based foods, such as beef, poultry, pork, fish, milk, yogurt, eggs, cheese, soya products, and some nuts and seeds. Phenylalanine is important for making thyroid hormones and neurotransmitters (nerve signalling chemicals), and maintains normal brain function.