Correct Answer: A. Tryptophan
UV absorbance of proteins at 280 nm is determined by aromatic amino acids, specifically tryptophan (Trp) and tyrosine (Tyr), which possess extended conjugated π-electron systems in their indole and phenol rings respectively. Alanine is a non-aromatic, aliphatic amino acid with no chromophoric properties. When alanine is replaced by tryptophan, the protein gains an indole ring with a maximum absorption peak at 280 nm (ε ≈ 5,500 M⁻¹cm⁻¹), dramatically increasing UV absorbance at this wavelength. This principle is exploited clinically in India for protein quantification using spectrophotometry (Bradford assay, BCA assay) and for monitoring protein concentration in diagnostic laboratories. The 280 nm wavelength is the gold standard for measuring protein concentration in clinical biochemistry because aromatic amino acids are the primary contributors to absorbance at this wavelength. Tryptophan has the highest molar extinction coefficient among all amino acids at 280 nm, making it the most significant contributor to protein UV absorbance.
Why the other options are wrong
B. Arginine — Arginine is a basic, positively charged amino acid with a guanidinium group. It is non-aromatic and lacks a conjugated π-electron system, so it does not absorb UV light at 280 nm. While arginine has a side chain that can interact electrostatically, it contributes negligibly to protein absorbance at 280 nm. This is a common distractor because students may confuse 'charged' with 'chromophoric.' C. Proline — Proline is an imino acid with a cyclic structure, but it is non-aromatic and lacks UV-absorbing chromophores. Although proline's ring structure is unique among amino acids, the π-electrons are not conjugated in a way that produces significant absorption at 280 nm. Proline is often tested as a distractor because of its structural distinctiveness, but it does not contribute to protein UV absorbance. D. Leucine — Leucine is a non-aromatic, hydrophobic, branched-chain amino acid with an aliphatic side chain. Like alanine, it lacks any chromophoric groups and does not absorb UV light at 280 nm. Leucine is a common distractor in biochemistry questions because it is frequently mentioned in discussions of protein structure and hydrophobic interactions, but it has no role in UV absorbance.
High-Yield Facts
- Tryptophan and tyrosine are the only amino acids that absorb UV light at 280 nm due to their aromatic rings.
- Tryptophan has ε ≈ 5,500 M⁻¹cm⁻¹ at 280 nm, making it the strongest UV absorber among all amino acids.
- Protein quantification at 280 nm relies on aromatic amino acid content and is the standard method in clinical laboratories across India.
- Non-aromatic amino acids (alanine, arginine, proline, leucine, glycine, serine) do not contribute to UV absorbance at 280 nm.
- Phenylalanine absorbs at 257 nm (not 280 nm), so it is not the primary contributor at 280 nm wavelength.
Mnemonics
AROMA at 280 Aromatic amino acids (Trp, Tyr) absorb at 280 nm. Remember: only amino acids with aromatic rings (benzene or indole) absorb at 280 nm. Trp > Tyr at 280 Trptophan has higher extinction coefficient than Tyrosine at 280 nm (5,500 vs 1,400 M⁻¹cm⁻¹). When replacing alanine, Trp gives the biggest boost.
NBE Trap
NBE pairs this question with non-aromatic amino acids (arginine, proline, leucine) to test whether students confuse 'structural uniqueness' or 'charge' with 'chromophoric properties.' The trap is that arginine is charged and proline is structurally distinct, but neither absorbs UV light at 280 nm.
Clinical Pearl
In Indian diagnostic laboratories, the 280 nm spectrophotometry method is routinely used to measure protein concentration in serum, CSF, and urine samples. A protein rich in tryptophan residues will show higher absorbance at 280 nm, which is why this wavelength is the gold standard for protein quantification in clinical biochemistry. Understanding this principle helps clinicians interpret protein assay results accurately.
_Reference: Harper's Biochemistry Ch. 3 (Amino Acids); Lehninger Principles of Biochemistry Ch. 4_