Fluorescence decay characteristics of indole compounds revealed by time-resolved area-normalized emission spectroscopy

Time-resolved fluorescence spectroscopy of tryptophan residue has been extensively applied to the studies on structure-function relationships of protein. Regardless of this, the fluorescence decay mechanism and kinetics of tryptophan residue in many proteins still remains unclear. Previous studies have demonstrated that conformational heterogeneity and relaxation dynamics are both involved in the peculiar multiexponential decay kinetics in subnanosecond resolution. In the present study, we characterized the fluorescence decay property of six indole compounds in glycerol by resolving the contribution of conformational heterogeneity and relaxation dynamics. We applied the time-resolved area-normalized fluorescence emission spectrum (TRANES) method for the fluorescence decay analysis. The results of TRANES, time-dependent shift of fluorescence spectral center of gravity, and fluorescence decay simulation demonstrated that the dielectric relaxation process independent of intrinsic rotamer/conformer and the individual fluorescence lifetime gives the peculiarity to the fluorescence decay of indole compounds. These results confirmed that TRANES and time-dependent spectral shift analysis are potent methods to resolve the origin of multiexponential decay kinetics of tryptophyl fluorescence in protein.