Chiral Centers in Aspartame's Structure

Steps:

• Aspartame is the dipeptide L-aspartyl-L-phenylalanine methyl ester.
• The aspartic acid residue has one chiral carbon at the alpha position, bonded to H, COOH, NH2, and CH2COOH.
• The phenylalanine residue has one chiral carbon at the alpha position, bonded to H, COOH, NH2, and CH2C6H5.
• The methyl ester and side chains introduce no additional chiral carbons.

Why B is correct:

• A chiral carbon requires four different substituents; aspartame's two alpha carbons meet this criterion, confirming two chiral centers.

Why the others are wrong:

• A: Ignores the second alpha carbon in phenylalanine.
• C: No third carbon with four distinct groups exists.
• D: Structure lacks four chiral carbons.

Final answer: B