Dipeptide Length from Amino Acid Residue Dimensions

Steps:

• Identify the standard contribution of one amino acid residue to polypeptide chain length in extended conformation: approximately 0.36 nm per residue.
• Note that a dipeptide forms by linking two amino acids via a peptide bond, sharing one bond and thus adding lengths additively with minimal overlap.
• Calculate total backbone length: roughly 2 residues × 0.36 nm, plus terminal groups extending about 0.48 nm total.
• Approximate full dipeptide length: 0.72 nm + 0.48 nm = 1.2 nm.

Why B is correct:

• Matches the standard biochemical measurement of ~1.2 nm for an extended dipeptide's end-to-end distance, based on X-ray crystallography data for peptide bonds and residue spacing.

Why the others are wrong:

• A underestimates by ignoring terminal extensions beyond the two Cα-Cα distances (~0.76 nm total).
• C overestimates, assuming helical conformation (0.15 nm rise per residue) rather than extended.
• D greatly overestimates, perhaps confusing with tetrapeptide length (~1.44 nm).

Final answer: B