When unfolded, essentially all groups become exposed to the solvent and the spectrum changes correspondingly. Complete unfolding should produce the same spectrum from all identical groups. The spectrum can thus be used to assess the extent of residual structure. Absorption spectroscopy is often used to follow the binding of various ligand molecules to the protein. So far we have restricted our discussion to the position of electronic levels.
Valuable information is obtained also from the time dependence of emission processes.
We return to Fig. 20.4 to discuss what happens after a photon has been absorbed. Fluorescence involves an allowed singlet-singlet transition and has characteristic lifetimes of the order of ns. Phosphorescence, however, occurs from a spin-triplet state and, since it is spin forbidden, the lifetime is very much longer, of the order of ms to s. According to the Frank-Condon principle the absorption process is so fast that the nuclei of the molecule do not have time to change their positions.