Dynamics of hydration water in sugars and peptides solutions

We analyzed solute and solvent dynamics of sugars and peptides aqueous solutions using extended depolarized light scattering (EDLS) and broadband dielectric spectroscopies (BDS). Spectra measured with both techniques reveal the same mechanism of rotational diffusion of peptides molecules. In the case of sugars, this solute reorientational relaxation can be isolated by EDLS measurements, whereas its contribution to the dielectric spectra is almost negligible. In the presented analysis, we characterize the hydration water in terms of hydration number and retardation ratio ξ between relaxation times of hydration and bulk water. Both techniques provide similar estimates of ξ. The retardation imposed on the hydration water by sugars is ~3.3 ± 1.3 and involves only water molecules hydrogen-bonded (HB) to solutes (~3 water molecules per sugar OH-group). In contrast, polar peptides cause longer range perturbations beyond the first hydration shell, and ξ between 2.8 and 8, increasing with the number of chemical groups engaged in HB formation. We demonstrate that chemical heterogeneity and specific HB interactions play a crucial role in hydration dynamics around polar solutes. The obtained results help to disentangle the role of excluded volume and enthalpic contributions in dynamics of hydration water at the interface with biological molecules.