Microscopic Heterogeneity Driven by Molecular Aggregation and Water Dynamics in Aqueous Osmolyte Solutions

Water dynamics are investigated in binary osmolyte-water mixtures, exhibiting a microscopic heterogeneity driven by molecular aggregation, on the basis of molecular dynamics (MD) simulation studies. The protecting osmolyte TMAO molecules in solution are evenly dispersed without the formation of noticeable osmolyte aggregates, while the denaturant TMU molecules aggregate readily, generating microscopic heterogeneity in the spatial distribution of component molecules in TMU-water mixtures. A combined study of MD simulation with graph theoretical analysis and spatial inhomogeneity measurement with h-values in the two osmolyte solutions revealed that the translational and rotational motions of water in the microheterogeneous environment of TMU-water mixtures are less hindered than those in the homogeneous media of TMAO-water mixtures. The analysis of the osmolyte-water H-bond lifetime in the binary solutions shows that destabilizing osmolyte TMU makes relatively weak osmolyte-water interaction, compared to that in protecting osmolyte TMAO, enabling the interplay of TMU-TMU or TMU-protein as well as TMU-water interaction. Taken together, the complementary contributions of the two hypotheses are proposed to elucidate the operating mechanism of the osmolyte on protein stability, encompassing a direct mechanism for the preferential interaction between the osmolyte and protein and an indirect mechanism for the modulation of the water structure and dynamics in the osmolyte solutions.