JEE Advanced 2024 Paper 1 Q03 Chemistry Biomolecules & Polymers Amino Acids, Peptides & Esters Medium

JEE Advanced 2024 Paper 1 · Q03 · Amino Acids, Peptides & Esters

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

[Figure: Structures of phenylalanine (Ph-CH2-CH(NH2)-COOH shown as H2N\\OH with side-chain Ph) and aspartic acid (HOOC-CH2-CH(NH2)-COOH shown as H2N\\OH with -COOH side chain) given for reference. The four options (A)-(D) show different connectivity / stereochemistry of the dipeptide methyl ester linking aspartic acid to phenylalanine via a peptide bond, with the C-terminus of phenylalanine as -COOMe.]

  1. A. (A) Aspartyl group with N-H of phenylalanine on the alpha-COOH of Asp; Phe -COOMe; side-chains Ph (on Phe) and -CH2COOH on alpha-carbon of Asp; H2N free on Asp.
  2. B. (B) HOOC-CH2-CH(NH2)-CO-NH-CH(CH2-Ph)-COOMe — peptide bond between alpha-COOH of L-aspartic acid and alpha-NH2 of L-phenylalanine; methyl ester on Phe carboxyl.
  3. C. (C) Same atoms as (B) but peptide bond formed via the beta-carboxyl (side-chain -CH2COOH) of aspartic acid instead of the alpha-COOH.
  4. D. (D) Methyl ester on aspartate carboxyl with phenylalanine free -COOH (regiochemistry inverted relative to natural aspartame).
Reveal answer + step-by-step solution

Correct answer:B

Solution

Aspartame is L-aspartyl-L-phenylalanine methyl ester: $H_2N\text{-}CH(CH_2COOH)\text{-}CO\text{-}NH\text{-}CH(CH_2Ph)\text{-}COOCH_3$. The peptide bond is between the $\alpha$-carboxyl group of aspartic acid and the $\alpha$-amino group of phenylalanine; the C-terminal carboxyl of phenylalanine is esterified as the methyl ester. The free $\beta$-carboxyl (side chain -CH2COOH) of aspartate remains. Option (B) depicts this correct connectivity. Option (C) uses the beta-COOH (isoaspartame, non-sweet); (A) and (D) have wrong regiochemistry of the methyl ester or stereo-mismatched arrangement.

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