Influence of the Microenvironment Dynamics of Tryptophan on Its Fluorescence Parameters at Different Temperatures

The temperature dependences of the fast and slow fluorescence decay components of aqueous tryptophan solution after freezing under actinic light and in the dark were investigated in the temperature range from –170 to 20°C. A model of the direct and reverse electronic transitions from the excited state to the ground state and to the state with charge transfer in a tryptophan molecule was used to perform quantitative analysis. Three main spectral regions of tryptophan fluorescence were shown; they differed in the behavior of the temperature dependences depicted for the rates of transition from the excited state of tryptophan to the state with charge transfer. It was shown that the dynamics of the hydrogen bond system plays a key role in this transition. The system of hydrogen bonding determines the nonlinear nature of tryptophan fluorescence in the selected spectral regions. The non-linear behavior of the fluorescence lifetime and fluorescence spectra with temperature change is determined by the character of interaction of tryptophan with water and ice. It was shown that temperature rearrangements play a critical role in hydrogen bonding structure of H₂O that surrounds a tryptophan molecule in the excited state.