Phenylalanine

(CC) Image: David E. Volk
Phenylalanine, one of the twenty common amino acids.

Phenylalanine, abbreviated Phe or F, is one of the twenty common $\alpha$-amino acids used by living organisms to build proteins. It is one of four aromatic amino acids with histidine, tyrosine and tryptophan. It is very hydrophobic is found almost exclusively buried within hydrophobic patches in protein structures. When aspartame was first introduced (as Nutrisweet) it caused the deaths of several people suffering from phenylketonuria due to the metabolic degradation of aspartame to phenylalanine, which phenylketonurics cannot metabolize.