Disaccharide topology induces slowdown in local water dynamics

Molecular level insight into water structure and structural dynamics near proteins, lipids, and nucleic acids is critical to the quantitative understanding of many biophysical processes. Unfortunately, understanding hydration and hydration dynamics around such large molecules is challenging because of the necessity of deconvoluting the effects of topography and chemical heterogeneity. Here we study, via classical all-atom simulation, the water structure and structural dynamics around two biologically relevant solutes large enough to have significant chemical and topological heterogeneity but small enough to be computationally tractable: the disaccharides kojibiose and trehalose. We find both molecules to be strongly amphiphilic (as quantified from normalized local density fluctuations) and to induce nonuniform local slowdown in water translational and rotational motions. Detailed analysis of the rotational slowdown shows that, while the rotational mechanism is similar to that previously identified in other aqueous systems by Laage, Hynes, and coworkers, two novel characteristics are observed: broadening of the transition state during hydrogen bond exchange (water rotation) and a subpopulation of water for which rotation is slowed because of hindered access of the new accepting water molecule to the transition state. Both characteristics are expected to be generic features of water rotation around larger biomolecules and, taken together, emphasize the difficulty in transferring insight into water rotation around small molecules to much larger amphiphilic solutes.