This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: NEET Jan 2020, Subject name: Biochemistry
Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength? 
Select and explain. 
A. Tryptophan, B. Arginine, C. Guanine, D. Lysine
A: Tryptophan
Answer: A: Tryptophan 
Explanation: Aromatic side chains are responsible for the UV absorption of most proteins, which have absorption maxima between 275 nm and 285 nm Basically, this absorption of UV-light is because of conjugated double bonds present in the ring. Aromatic amino acids: Tryptophan Tyrosine Phenylalanine Absorption ismaximum for tryptophan. Since nearly all proteins contain aromatic amino acids, the amount of light absorbed at 280 nm by a protein is used as an indirect measure of protein concentration.