Which amino acids contribute to protein absorbance at 280 nm?

• A. Tryptophan, Tyrosine, Phenylalanine
• B. Leucine, Isoleucine, Valine
• C. Methionine, Leucine, Proline
• D. Asparagine, Glutamine, Serine

Answer: (A)

Protein absorbance at 280 nm comes from aromatic amino acids that have conjugated ring systems capable of absorbing UV light. Among the amino acids, tryptophan has the strongest absorption, tyrosine also absorbs significantly, and phenylalanine contributes as well, though to a lesser extent. The other listed amino acids—leucine, isoleucine, valine, methionine, proline, asparagine, glutamine, and serine—do not have aromatic rings, so they do not meaningfully absorb at this wavelength. Therefore, the major contributors to absorbance at 280 nm are tryptophan, tyrosine, and phenylalanine.