Temperature Dependence of the Nonlinear Dynamics of the Deactivation of Excited States of Tryptophan in Various Media

The authors’ research on the dynamics of the excited states of tryptophan in various solvents as a function of temperature (–170 to +20°С) is presented. The antibatic behavior of the temperature dependences of the decay times of two components (fast and medium) of tryptophan fluorescence is found in the temperature range from –60 to +10°C. The third, slow, component shows a weak dependence on temperature. The antibatic behavior of the decay times of two components of the fluorescence kinetics is modeled under the assumption that, in a certain temperature range, some of the tryptophan molecules in the excited state pass from the short-wavelength rotamer B-form, which has a short fluorescence lifetime, to the long-wavelength rotamer R-form, with an intermediate fluorescence lifetime. To explain the observed changes in the spectra and duration of tryptophan fluorescence depending on temperature, a new model of the transitions between the excited and ground states using the charge transfer state (CTS), which takes into account the nonlinear nature of the dynamics of these transitions, is also developed. In these processes, an important role is played by the interaction of tryptophan molecules with its microenvironment and the rearrangements in the system of hydrogen bonds in the environment of the tryptophan molecule. Three main spectral regions of tryptophan fluorescence, which differ in the behavior of the temperature dependences of the rates of transition from the excited state of tryptophan to CTS, are distinguished. The key role of the dynamics of the hydrogen bond system, which determine the nonlinear nature of the change in tryptophan fluorescence parameters in the selected spectral regions, is shown.