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
Substitution of which one of the following amino acids in place of alanine would increase the absorbance of proteins at 280 mm: 
Select and explain. 
A. Leucine, B. Arginine, C. Tryptophan, D. Protein
C: Tryptophan
Answer: C: Tryptophan 
Explanation: The only amino acids that may absorb light are Aromatic amino acids i.e. Tryptophan, Tyrosine, Phenylalanine & Histidine- Most a.a do not absorb visible light & are thus colourless- Aromatic aminoacids absorb ultraviolet light (250- 290 nm).- Tryptophan makes major contribution to ability of most proteins to absorb light in the region of 280 nm.