Aromatic residues and protein spectroscopy: strongest UV absorption at 280 nm Which pair of amino acids together accounts for the strongest absorbance signal near 280 nm used to estimate protein concentration?

Introduction / Context:
UV absorbance at 280 nm is a standard method for estimating protein concentration. The signal arises primarily from aromatic side chains with strong electronic transitions in this region.

Given Data / Assumptions:

• We consider typical amino acid chromophores.
• Assume neutral pH and standard pathlengths for spectrophotometry.

Concept / Approach:
Tryptophan (Trp) contributes the largest molar absorptivity at 280 nm, followed by tyrosine (Tyr). Phenylalanine absorbs mainly closer to 257 nm and much more weakly at 280 nm. Disulfide-linked cystines also absorb but contribute less than Trp or Tyr.

Step-by-Step Solution:
Identify primary chromophores: Trp > Tyr > Phe.Select the pair that maximizes 280 nm signal: Trp and Tyr.Exclude pairs lacking strong aromatic transitions at 280 nm.

Verification / Alternative check:
Common protein assays (A280) and extinction coefficient calculations weight Trp and Tyr residues most heavily.

Why Other Options Are Wrong:
Threonine, histidine, cysteine, asparagine, proline, glycine, serine lack strong 280 nm absorbance; phenylalanine’s contribution is modest.

Common Pitfalls:
Overestimating phenylalanine’s role at 280 nm; forgetting that buffer components can also absorb UV.

Final Answer:
Trp & Tyrosine.