Considerations for the Construction of the Solvation Correlation Function and Implications for the Interpretation of Dielectric Relaxation in Proteins

The dielectric response of proteins is conveniently measured by monitoring the time-dependent Stokes shift of an associated chromophore. The interpretation of these experiments depends critically upon the construction of the solvation correlation function, C(t), which describes the time-dependence of the Stokes shift and hence the dielectric response of the medium to a change in charge distribution. We provide an analysis of various methods of constructing this function and review selected examples from the literature. The naturally occurring amino acid, tryptophan, has been frequently used as a probe of solvation dynamics in proteins. Its nonexponential fluorescence decay has stimulated the generation of an alternative method of constructing C(t). In order to evaluate this method, we have studied a system mimicking tryptophan, The system is comprised of two coumarins (C153 and C152) having different fluorescence lifetimes but similar solvation times. The coumarins are combined in different proportions in methanol to make binary probe mixtures. We use fluorescence upconversion spectroscopy to obtain wavelength-resolved kinetics of the individual coumarins in methanol as well as the binary mixtures of 75:25, 50:50, and 25:75 of C153:C152. The solvation correlation functions are constructed for these systems using different methods and are compared.