Aspartame, an artificial sweetener, is a dipeptide aspartyl phenylalanine methyl ester. The structure of aspartame is

Aspartame is named L-aspartyl-L-phenylalanine-1-methyl ester. The systematic name already tells us how every group is connected, so we decode it step by step.

1. “L-aspartyl” means the N-terminal amino acid is L-aspartic acid. Aspartic acid contains
  • an α-amino group $$NH_2$$ attached to $$CH$$ (the α-carbon),
  • an α-carboxyl group $$COOH$$, and
  • a side-chain carboxyl group $$CH_2COOH$$.

2. “L-phenylalanine” is the second amino acid. Its α-carbon carries
  • an α-amino group (but this is consumed in peptide bonding),
  • a benzyl side chain $$CH_2\!\!\!- \!\!\!\bigcirc$$, and
  • an α-carboxyl group $$COOH$$.

3. “Methyl ester” tells us that the carboxyl group of phenylalanine is esterified with methanol, converting $$COOH$$ into $$COOCH_3$$.

Putting these facts together:

• The peptide bond must be between the α-carboxyl of aspartic acid and the α-amino of phenylalanine (standard N→C direction in peptides).
• The side-chain carboxyl of aspartic acid remains free as $$COOH$$.
• The phenylalanine α-carboxyl is not a free acid; it is present as $$COOCH_3$$.

Only one of the given structures shows simultaneously
  ✔ a peptide linkage $$-CO-NH-$$ between the correct α-centres,
  ✔ a free second $$COOH$$ on the aspartyl side chain, and
  ✔ the $$COOCH_3$$ ester on the phenylalanine end,
with both chiral centres having the L-configuration. That structure is shown in Option B.

Hence, the correct structure of aspartame is given in Option B.

Answer: Option B which is: the L-aspartyl-L-phenylalanine-1-methyl ester structure