To different degrees, all aromatic amino acids absorb ultraviolet light.
Proteins contain several chromophores that absorb light in the ultra-violet and infra red regions. Many also display fluorescence. The most important chromophores are the aromatic rings of Phe, Tyr and Trp. UV absorbance and fluorescence are useful probes of structure and structural changes. This is due to the fact that chromophores display shifted spectra upon increasing or decreasing polarity of their environment, with changes in wavelength of maximum absorbance (lambda max) and molar extinction coefficient possible.