Aromatic amino acids (phenylalanine, tryptophan, and tyrosine) absorb UV light at a maximum of 280 nm because the pi electrons in their aromatic rings can absorb photons. That is because benzene, the core of a phenyl group, absorbs UV light as well. Amino acids do not absorb visible light and thus are colorless. However, tyrosine, phenylalanine, and especially tryptophan absorb high-wavelength (250-290 nm) ultraviolet light. Because it absorbs ultraviolet light about ten times more efficiently than phenylalanine or tyrosine, tryptophan makes the major contribution to the ability of most proteins to absorb light in the region of 280 nm. Ref: Kennelly P.J., Rodwell V.W. (2011). Chapter 3. Amino Acids & Peptides. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds),Harper's Illustrated Biochemistry, 29e.