Fluorescence Lifetime Distributions of Single Tryptophan Proteins: A Protein Dynamics Approach

Abstract

The investigation of fluorescence from proteins has always been regarded as a tool to study protein conformations and dynamics. The structural information is associated with the sensitivity of the emission spectrum to the exposure of tryptophan and tyrosine residues to the solvent. Dynamic information was obtained by the analysis of the decay of the emission anisotropy and other excited state processes such as quenching by fast diffusing molecules or resonance energy transfer. Fluorescence lifetime measurements has been used to detect different protein conformations: the values of the decay rates and preexponential factors has been associated with a particular conformation and with the relative population in each conformation. However, the accurate study of the emission from several single tryptophan proteins has shown that the number of lifetime components can be large and the identification with different protein conformations is difficult. The sensitivity of indole fluorescence to a wide variety of environmental conditions is well recognized (1) and is the principal factor in the diversity of fluorescence observed between peptides and proteins. Solvation is certainly a major factor in determining the nature of tryptophan fluorescence in proteins, but there are many other moieties intrinsic to the protein structure and/or excited state processes which can affect the fluorescence lifetime markedly (2,3).