The fluorescence of a protein is determined by interactions of the fluorophore with the protein matrix and with solvent. The usefulness of fluorescence spectroscopy is predicated on the ability to interpret the fluorescence signal in terms of specific molecular interactions. Although it is commonly held that we understand how such interactions influence fluorescence, this is not substantiated by available data. This not surprising given (i) the complexity of the environs of fluorophores in proteins; (ii) our minimal understanding of the dynamics of the protein matrix and of water interacting with the understanding of the dynamics of the protein matrix and of water interacting with the protein matrix; (iii) the lack of adequate theory describing the excited state of fluorophores, (iv) the lack of information on the effects of electrostatic interactions on fluorophore photophysics, and (v) the uncertainty (engendered by i-iv) of being able to devise good physical models to explain the experimental data. Any physical model must be able to explain all of the fluoresence properties of a protein (ie. quantum yield, emission spectra, r(t), and fluorescence quenching). Our approach to correlating protein structure and intrinsic (trypotophan) luminescence requires concurrent use of (a) steadystate and time (picosecond) resolved fluorescence spectroscopy (and other spectroscopic methods where necessary) to determine the fluorescence properties of proteins of known crystal structure, each containing a single trp residue but exhibiting different fluorescence properties; (b) Molecular dynamics simulations - including activated molecular dynamics calculations - to assess trp side chain motion, water accessibility and dynamics, and electrostatic interactions with the trp residue; (c) molecular graphics depictions; (d) Semi-empirical calculations on the photophysics of indole and other commonly used fluorophores in an attempt to provide at least a semiquantitative assessment of environmental effects on fluorescence. The principal initial goal is to examine further the views that interpretation of fluorescence lifetimes in terms of distribution of states is valid and that whereas emission spectrum reflects the effects of a static field in the trp environs. Lastly, we suggest that dipolar relaxation in proteins probably arises from re-orientation of solvent molecules adsorbed to the protein matrix adjacent to the fluorophore.
Showing the most recent 10 out of 67 publications
