Which amino acids strongly absorb at 280 nm?
Protein absorption near 280 nm originates exclusively from aromatic side chains possessing conjugated π electrons capable of π→π* excitation. Phenylalanine absorbs weakly around 257 nm, tyrosine near 274 nm with ε approximately 1490 M-1 cm-1 due to phenolic hydroxyl, tryptophan dominates near 280 nm with ε about 5500 M-1 cm-1 due to indole ring delocalization. Aliphatic residues glycine, alanine, valine, leucine and acidic aspartate, glutamate lack delocalized electrons, absorbing only below 220 nm from peptide backbone. Thus A280 quantifies proteins containing these aromatic residues and permits concentration estimation using combined extinction coefficients.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.