Aspartame. Molecular model of the artificial sweetener aspartame. Atoms, represented as tubes, are colour-coded; carbon (blue), hydrogen (white), oxygen (red) and nitrogen (dark blue). Its chemical name is L-alpha-aspartyl-L-phenylalanine methyl ester. Aspartame is about 180 times sweeter than sugar. It has no energy value and is therefore used as a sugar substitute in diet food, drinks and sweeteners. It contains the amino acid phenylalanine and must carry a warning on the product packaging. This is because people who suffer from the condition phenylketonuria cannot metabolise phenylalanine, and over-consumption can lead to mental retardation.