Red-edge-excitation fluorescence spectroscopy of indole and tryptophan

Studies on the dependence of indole and tryptophan fluorescence emission spectra on excitation wavelength, ^ex, show that the emission shifts to longer wavelengths for red-edge excitation in different solid and viscous solvents. In solid systems the spectral shifts for excitation in the range from 290 to 310 nm can reach tens of nm, and they are more significant than changes of ^ex. In a viscous medium the magnitude of this effect is shown to be directly related to the dipole-reorientational relaxation of solvent molecules in the environment of the chromophore, which allows the relaxation times to be estimated. The method involves simple steady-state measurements of fluorescence spectra at the maximum and at the red edge of the absorption band. Since it is not necessary to obtain information on the fluorescence spectra of completely relaxed states, this method for the estimation of relaxation times may have advantages in studies of proteins compared with the conventional relaxation shift method, and may produce complementary information to that obtained by nanosecond time-resolved spectroscopy.