Time-resolved fluorescence of tryptophan in biophysical chemistry and pharmaceutical research – the pleasures and nightmares dealing with nature’s own fluorophore

Time-resolved (TR) intrinsic fluorescence of tryptophan (Trp) provides a wealth of information on the structure and localization of proteins and peptides and their interactions with one another, with drugs, lipid membranes, lipid- and surfactant-based drug delivery systems, et cetera. Intrinsic Trp eliminates the need for labeling and avoids the perturbation of the system by the label; introduced Trp is a rather conservative and small label compared to others. Whereas custom-tailored fluorophores are often optimized for a special technique, Trp can be employed to monitor a wide variety of effects. We address interactions of Trp with surrounding molecules, dynamic quenchers and Förster resonance energy transfer (FRET) acceptors that affect the fluorescence decay. Speed and range of angular motion of Trp are characterized by TR anisotropy. Electrostatic interactions of Trp with charged and polar molecules, including water, are monitored by decay-associated spectra (DAS) or TR emission spectra (TRES) and quantified in terms of TR shifts of the spectral center of gravity. This versatility is a great advantage and, at the same time, comes with a complexity of the behavior that can render it a challenge to interpret the data in detail properly. This review provides an overview of applications of TR fluorescence of Trp bulk samples in biomolecular, biophysical, and pharmaceutical studies. The aim is not only to point out the diversity of the read-out of these techniques, but also critically examine their current use. Therefore, we identify most common technical pitfalls and evaluate the degree of reliability of the interpretational approaches. This should aid a more extensive and meaningful use of TR fluorescence of Trp.