Dynamics of uncrystallized water and protein in hydrated elastin studied by thermal and dielectric techniques

Dynamics of uncrystallized water and protein was studied in hydrated pellets of the fibrous protein elastin in a wide hydration range (0 to 23wt.%), by differential scanning calorimetry (DSC), thermally stimulated depolarization current technique (TSDC) and dielectric relaxation spectroscopy (DRS). Additionally, water equilibrium sorption-desorption measurements (ESI) were performed at room temperature. The glass transition of the system was studied by DSC and its complex dependence on hydration water was verified. A critical water fraction of about 18wt.% was found, associated with a reorganization of water in the material. Three dielectric relaxations, associated to dynamics related to distinct uncrystallized water populations, were recorded by TSDC and DRS. The low temperature secondary relaxation of hydrophilic polar groups on the protein surface triggered by hydration water for almost dry samples contains contributions from water molecules themselves at higher water fractions (ν relaxation). This particular relaxation is attributed to water molecules in the primary and secondary hydration shells of the protein fibers. At higher temperatures and for water fraction values equal to or higher than 10wt.%, a local relaxation of water molecules condensed within small openings in the interior of the protein fibers was recorded. The evolution of this relaxation (w relaxation) with hydration level results in enhanced cooperativity at high water fraction values, implying the existence of "internal" water confined within the protein structure. At higher temperatures a relaxation associated with water dynamics within clusters between fibers (p relaxation) was also recorded, in the same hydration range.