Comparison of spectral and temporal fluorescence parameters of aqueous tryptophan solutions frozen in the light and in the dark

Abstract

Comparative investigation of temperature dependences of the fast and slow fluorescence decay components in an aqueous solution of tryptophan molecules, frozen in the light and in the dark, was made in the range of –170 °C to + 20 °C. A quantitative analysis of the model of direct and reverse electronic transitions in tryptophan molecule from an excited state to the ground state and to the charge transfer state (CTS) was carried out. Three main spectral regions of tryptophan fluorescence are distinguished that differ in behavior of temperature dependences of the transition rates from the excited state of tryptophan to CTS. The key role of the hydrogen bond system dynamics determining the nonlinear behavior of tryptophan fluorescence parameters in the selected spectral regions is shown. The nonlinear behavior of the fluorescence lifetime, position and shape of fluorescence spectra with changing temperature is determined by tryptophan interaction with an aqueous medium. Besides, the rate of solvation shift of fluorescence spectra is observed. The important role of temperature rearrangements in the system of hydrogen bonds of the water solution surrounding the tryptophan molecule in the excited state is shown.The obtained results can be used to explain the key role of hydrogen bonds in the processes of charge transfer and energy migration in biological systems.