Aspartame. Molecular model of the sugar substitute Aspartame (Nutrasweet, formula: C14H18N2O5). Atoms are spherical and colour-coded: carbon (blue), hydrogen (white), oxygen (red) and nitrogen (light blue). Aspartame is a registered trademark for L-alpha- Aspartyl-L-phenylalanine methyl ester. Its main uses are in canned drinks and foods, and as a consumer-added sweetener in the form of tablets and granules. 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 can not metabolise phenylalanine, and over-consumption can lead to mental retardation.