Practice question

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.