This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Replacing alanine by which amino acid will increase UV absorbances of protein at 280 nm wavelength 
Select and explain. 
A. Leucine, B. Proline, C. Arginine, D. Tryptophan
D: Tryptophan
Answer: D: Tryptophan 
Explanation: Aromatic amino acids absorb UV light, which has absorption maxima between 275 nm-285 nm.
Tryptophan has greater absorption in this region than other two aromatic amino acids (tyrosine, phenylalanine).
Since nearly all proteins contain aromatic amino acids, the amount of light absorbed at 280 nm by protein is used as an indirect measure of protein concentration.