Fluorescence line narrowing spectroscopy: a tool for studying proteins.

Perhaps the most important contribution of FLN is that it provides an experimental approach to relate physical changes in the protein to predicted dynamical behavior. It is clear that the sample is inhomogeneously broadened in a continuous manner, consistent with the damped motion of proteins. At the same time configurational substates can be selected, suggesting that there is indeed a hierarchy of protein motion and structure. As yet, identification of the structure, and relating it to the spectra, has not been achieved. It is clear that the electric field exerted by neighboring atoms shifts the electronic transition, and the inhomogeneity is greater when the surrounding disorder is greater. The inhomogeneity for the chromophore in the protein is dependent on the protein conformation and is intermediate between that of a crystal and a glass. The phonon coupling also depends on the chromophore and the protein. Fluorescence line narrowing provides in addition ground- and excited-state vibrational frequencies, thereby allowing for structural differences between the excited-state and the ground-state molecule to be detected.