The property of protein to absorb UV rays of light is due to:

Aromatic amino acids:- Amino Acid Absorb UV Light:  Amino Acids which absorb 250-290 nm (Maximum at 280 nm) UV light are tryptophan, phenylalanine, tyrosine. Maximum absorption of UV light by tryptophan.  Remember:  Aromatic amino acids absorb UV light.  Amino acids are colorless because, they do not absorb visible light.