Mean square fluctuations of hydrogen atoms and water-biopolymer interactions in hydrated saccharides

We have used the elastic neutron scattering technique to investigate the dynamics of the two main saccharidic components of starch: amylose and amylopectin. The measurements were carried out in the temperature range of 20 to 320 K and at different hydration levels from the dry state up to 0.47 g saccharide/g D2O. In the dry samples, the atomic dynamics is harmonic up to approximately 300 K. In the hydrated samples a "glass-like" transition leading to an anharmonic dynamics is observed. The onset of the anharmonicity occurs at temperatures that increase from similar to 180 K to 260 K upon decreasing hydration from 0.5 to 0.1 g saccharide/g D2O. This behavior is qualitatively similar to that observed in hydrated globular proteins, but quantitative differences are present. Assuming a simple asymmetric double-well potential model, the temperature and hydration dependence of the transition have been described in terms of few physical parameters.