Analysis of time-dependent red shifts in fluorescence emission from tryptophan residues in proteins.

Instantaneous fluorescence emission spectra measured at different times after excitation often shift to the red as the delay between the excitation pulse and fluorescence detection is increased. In the case of Trp fluorescence in proteins, the time-dependent red shift (TDRS) may have its origins in relaxation, heterogeneity, or a mixture of the two. In those cases where it is possible to rule out the contribution of heterogeneity, the TDRS can be used to study nonequilibrium relaxation dynamics of the protein matrix and the solvent on the picosecond and nanosecond time scales. Here we describe the experimental and computational procedures involved in recording spectrally and time-resolved fluorescence, detecting heterogeneity, and extracting information about protein/solvent relaxation dynamics.