Question 3

Aspartame is chemically known as $L$-aspartyl-$L$-phenylalanine methyl ester. It is a dipeptide where the $\alpha$-carboxylic acid group of aspartic acid forms a peptide bond with the amino group of phenylalanine, and the carboxylic acid group of the phenylalanine residue is esterified with methanol. Structure (A) correctly represents the $\alpha$-linkage: $H_2N-CH(CH_2COOH)-CONH-CH(CH_2Ph)-COOCH_3$.