Aspartame is an artificial sweetener sold under the trade name NutraSweet^TM. Aspartame is the methyl ester of a dipeptide formed from l‑aspartic acid and l-phenylalanine and can be summarized as Asp-Phe-OCH₃. Surprisingly, the analogous ethyl ester (Asp-Phe-OCH₂CH₃) is not sweet. If either l residue is replaced with its enantiomeric d residue, the resulting compound is bitter instead of sweet.

(a) Draw a bond-line structure of aspartame.

(b) Draw a bond-line structure for each of the three bitter stereoisomers of aspartame.