Residue mass comparison: Among the twenty standard amino acids, which has the greatest molecular mass and is therefore the heaviest?

Introduction / Context:
Residue mass affects mass spectrometry interpretation, peptide mobility, and protein engineering. Recognizing the heaviest common residue helps in rapid checks of theoretical peptide masses.

Given Data / Assumptions:

• Compare standard average or monoisotopic masses of side chains.
• Aromatic residues (Trp, Tyr, Phe) are heavier than aliphatic residues.

Concept / Approach:
Tryptophan has an indole ring system, giving it the largest side chain and greatest mass among canonical residues. Tyrosine and phenylalanine follow, with histidine lighter due to the imidazole ring.

Step-by-Step Solution:
Recall relative masses: Trp > Tyr > Phe > His, approximately.Identify the top candidate: tryptophan.Select ‘‘Tryptophan’’.

Verification / Alternative check:
Mass tables: Trp ~204 Da; Tyr ~181 Da; Phe ~165 Da; His ~155 Da (average masses; monoisotopic similar ordering).

Why Other Options Are Wrong:

• Phenylalanine and tyrosine are heavy but not heavier than tryptophan.
• Histidine is lighter than the three listed aromatics.

Common Pitfalls:
Confusing hydrophobicity with mass; assuming tyrosine’s oxygen increases mass above tryptophan (the indole ring still dominates).

Final Answer:
Tryptophan