Structure of Aspartame Artificial Sweetener Dipeptide Ester

To determine the correct chemical structure of aspartame, we analyze its IUPAC/chemical designation, connectivity, and stereochemistry.

1. Structure and Connectivity of Aspartame

Aspartame is chemically defined as L-$\alpha$-aspartyl-L-phenylalanine methyl ester:

• N-terminus (Aspartic Acid unit):
  • Contains a free $\alpha$-amino group ($-\text{NH}_2$).
  • The peptide bond ($-\text{C(=O)}-\text{NH}-$) is formed using the $\alpha$-carboxyl group attached directly to the $\alpha$-carbon.
  • The side-chain carboxyl group ($-\text{CH}_2\text{COOH}$) remains free.
• C-terminus (Phenylalanine Methyl Ester unit):
  • The $\alpha$-amino group forms the peptide bond with the aspartyl moiety.
  • The side chain is a benzyl group ($-\text{CH}_2\text{Ph}$).
  • The $\alpha$-carboxyl group is esterified with methanol to form a methyl ester ($-\text{COOMe}$).

2. Evaluation of Options by Connectivity

• Option (A): The peptide linkage is formed via the $\beta$-carboxyl group ($-\text{CH}_2\text{COOH}$) of aspartic acid rather than its $\alpha$-carboxyl group ($\beta$-aspartyl isomer). Thus, this is incorrect.
• Option (C): The order of amino acids is reversed; phenylalanine is at the N-terminus and aspartic acid is at the C-terminus. Thus, this is incorrect.
• Option (D): The methyl ester is located on the $\beta$-carboxyl group of aspartic acid instead of the carboxyl group of phenylalanine. Thus, this is incorrect.
• Option (B):
  • Aspartic acid is at the N-terminus with a free $\alpha$-amino group and a free $\beta$-carboxyl group ($-\text{CH}_2\text{COOH}$).
  • The peptide bond is formed between the $\alpha$-carboxyl of aspartic acid and the amino group of phenylalanine.
  • The C-terminal carboxyl group of phenylalanine is correctly converted to a methyl ester ($-\text{COOMe}$).

3. Verification of Stereochemistry

Both amino acids in natural aspartame must retain their $L$-configuration ($(S)$-stereocenter at the $\alpha$-carbons):

1. Aspartic Acid Residue ($\alpha$-carbon):

   • Priority: $-\text{NH}_2 > -\text{C(=O)NH}- > -\text{CH}_2\text{COOH} > -\text{H}$.
   • In structure (B), the $-\text{CH}_2\text{COOH}$ group is on a wedge pointing upwards, matching the reference structure of $L$-aspartic acid.

2. Phenylalanine Residue ($\alpha$-carbon):

   • Priority: $-\text{NH}- > -\text{COOMe} > -\text{CH}_2\text{Ph} > -\text{H}$.
   • In structure (B), the $-\text{CH}_2\text{Ph}$ group is on a dashed bond pointing downwards. Counting priorities $1 \to 2 \to 3$ gives a clockwise direction with hydrogen on a wedge (pointing towards the viewer), which corresponds to the $(S)$-configuration ($L$-phenylalanine).

Hence, Option (B) is the correct structure of aspartame.