The dynamical behavior of hydrated glutathione: a model for protein-water interactions.

The reliability of tripeptide glutathione as an excellent model for protein-water interactions is tested by means of broadband dielectric spectroscopy. Measurements performed on aqueous solutions with different water contents show a surprisingly rich relaxation map that strongly resembles those observed for more complex protein macromolecules. At variance with what is normally observed for solutions of hydrophilic compounds with similar molecular weights, the presence of at least two water-related processes is detected. The faster one is symmetric, has an Arrhenius temperature dependence with an activation energy E(A) = 0.45 +/- 0.05 eV and is attributed to water dipole reorientation. The slower one undergoes a clear dynamical change from a non-Arrhenius to an Arrhenius temperature dependence when crossing the calorimetric glass transition temperature of the solution from high to low values. This last process is proposed to be due to the dynamics of strongly-hydrated glutathione components, such as carboxyl or aminic groups.