Intrinsic protein fluorescence — Fluorescence from native proteins in the near-UV arises predominantly from which aromatic amino-acid residue?

Introduction / Context:
Intrinsic protein fluorescence is a powerful probe of protein folding, conformational changes, and ligand binding. It mainly originates from aromatic residues excited in the near-UV, with emission sensitive to the local environment.

Given Data / Assumptions:

• Three aromatic residues exist in proteins: phenylalanine, tyrosine, and tryptophan.
• Quantum yield and sensitivity differ dramatically among them.
• We assume typical excitation around 280 nm and emission monitoring near 320–350 nm.

Concept / Approach:
Tryptophan has the highest quantum yield and the most environment-sensitive emission maximum, often dominating total protein fluorescence even when present at low abundance. Tyrosine fluoresces weakly and is often quenched or energy-transferred to tryptophan. Phenylalanine has very low quantum yield and minimal contribution under standard conditions. Arginine is non-aromatic and non-fluorescent in this context.

Step-by-Step Solution:

Verification / Alternative check:
Empirical spectra of purified proteins show that mutating or quenching tryptophan residues drastically reduces fluorescence, validating its dominant role.

Why Other Options Are Wrong:

• Tyrosine: contributes, but far weaker and often quenched.
• Phenylalanine: weakest fluorescence among aromatics.
• Arginine: aliphatic, non-aromatic; does not fluoresce in the near-UV.

Common Pitfalls:
Assuming the most abundant aromatic determines intensity; a single buried tryptophan can dominate over multiple tyrosines due to higher quantum yield.

Final Answer:
Tryptophan