Temperature Induced Dynamical Transition of Biomolecules in Polarizable and Nonpolarizable TIP3P Water.

Temperature induced dynamic transition (DT), associated with sharp rise in molecular flexibility, is well-known to be exhibited between 270-280 K in glycerol to 200-230 K in hydrated biomolecules and is controlled by diffusivity (viscosity) of the solvation layer. In the molecular dynamics (MD) community, especially for water as a solvent, this has been an intense area of research despite decades of investigations. However, in general, water in these studies are described by empirical nonpolarizable force fields in which electronic polarizability are treated implicitly with effective charges and related parameters. This might have led to the present trait of discovery that DT of biomolecules, irrespective of the potential functions for water models used, occur within a narrow band of temperature variation (30-40 K). Whereas, water molecule in a biomolecular surface and in bulk is polarized differently, and therefore explicit treatment of water polarizability would be a powerful approach towards the treatment of hydration water, believed to cause the DT manifestation. Using MD simulations, we investigate the effects of polarizable water on the DT of biomolecules and the dynamic properties of hydration water. We choose two types of solutes: globular protein (lysozyme) and more open and flexible RNAs (β-hairpin and a riboswitch) with different nature of hydrophilic sites than proteins in general. We found that characteristic temperature of DT (T_DT) for the solutes in polarizable water is always higher than that in its nonpolarizable counterpart. In particular, for RNAs the variations are found to be ~ 45 K between the two water models, whereas for more compact lysozyme it is only ~ 6 K. The results are discussed on the light of enormous increase in relaxation times of a liquid upon cooling in the paradigm of dynamic switchover in hydration water with liquid-liquid phase transition, derived from the existence of the second critical point. Our result supports the idea that structures of biomolecules and their interactions with the hydration water determines T<sub>DT</sub and provides evidence for the decisive role of polarizable water on the onset of DT, which has been hitherto ignored.