Characterization of hydration water in supercooled water-trehalose solutions: The role of the hydrogen bonds network

The structural and dynamical properties of hydration water in aqueous solutions of trehalose are studied with molecular dynamics simulation. We simulate the systems in the supercooled region to investigate how the interaction with the trehalose molecules modifies the hydrogen bond network, the structural relaxation, and the diffusion properties of hydration water. The analysis is performed by considering the radial distribution functions, the residence time of water molecules in the hydration shell, the two body excess entropy, and the hydrogen bond water-water and water-trehalose correlations of the hydration water. The study of the two body excess entropy shows the presence of a fragile to strong crossover in supercooled hydration water also found in the relaxation time of the water-water hydrogen bond correlation function, and this is in agreement with predictions of the mode coupling theory and of previous studies of the oxygen-oxygen density correlators [A. Iorio et al., J. Mol. Liq. 282, 617 (2019); Sci. China: Phys., Mech. Astron. 62, 107011 (2019)]. The water-trehalose hydrogen bond correlation function instead evidences a strong to strong crossover in the relaxation time, and this crossover is related to a trehalose dynamical transition. This signals the role that the strong interplay between the soluted molecules and the surrounding solvent has in determining the dynamical transition common to both components of the system that happens upon cooling and that is similar to the well known protein dynamical transition. We connect our results with the cryoprotecting role of trehalose molecules.