Some experiments have witnessed increasing decoupling of viscosity from the translational self-diffusion of supercooled water with decreasing temperature. While theory and computer simulation studies indicated the jump translation of the molecules as a probable origin of the above decoupling, a precise quantitative estimation is still lacking. Through a molecular dynamics (MD) simulation study, along with careful consideration of translational jump motion, we have found the most definite proof of increasing relevance of translational jump diffusion in the above decoupling phenomena. By separating out the jump-only diffusion contribution from the overall diffusion of the water, we obtain the residual diffusion coefficient, which remains strongly coupled with the viscosity of the medium at the whole temperature range, including supercooled regime. These new findings can help to elucidate many experimental studies featuring molecular transport properties, where strong diffusion-viscosity decoupling comes into the picture. 3There are intriguing properties of supercooled water, including a strong decoupling between its viscosity and the diffusion of the molecules. Some experimental studies [1][2][3] -including that by Dehaoui et al.[4]-has revealed an increasing decoupling of viscosity from the water translational diffusion coefficient upon cooling. This indicates a gradual breakdown of the Stokes-Einstein (SE) relation ( with decreasing temperature. In contrast, the rotational diffusion D r remains coupled with for a wide range of temperature, which implies the validity of the Stokes-Einstein-Debye (SED) relation. Similar decoupling between D t and  was alsoreported earlier in other molecular glass forming liquids.[5-13] The SE relation is obeyed at sufficiently high temperature, but severely breaks down around 1.3T g (T g is the glass transition temperature). On the contrary, the rotational diffusion of the molecular glass forming liquid and the medium viscosity remain hydrodynamically coupled even at the temperature very close to T g .Deeply supercooled liquids have spatially heterogeneous dynamics, which have been confirmed by various experiments (e.g., see Refs. 5,6,[14][15][16][17] and computer simulation studies (e.g., see . A number of computer simulation studies have indicated that the emerging spatiotemporal heterogeneity in supercooled water and other supercooled liquids has connection with the increasing violation of the SE relation with decreasing temperature. [23][24][25][26][27][28][29][30] Recently, two of us have shown that the rotation assisted translational movement of solvent water around a nonpolar solute induces translational jump-diffusion of a tracer from one solvent cage to another in supercooled water.[23]Even though the prior studies have implied the pivotal role of translational jumpdiffusion for the breakdown of the SE relation in supercooled water, a quantitative estimation of the explicit contribution of the jump-only diffusion D Jump (diffusion due to jump only motion) is still missing...

A recent experiment has measured the viscosity of water down to approximately 244 K and up to 300 MPa. The correct viscosity and translational diffusivity data at various temperature–pressure (T–P) state points allowed for checking the validity of the Stokes–Einstein (SE) relation, which accounts for the coupling between translational self-diffusion and medium viscosity. The diffusion–viscosity decoupling increases with decreasing temperature, but the increasing pressure reduces the extent of the decoupling. Earlier simulation studies explained the breakdown of the SE relation in terms of the location of the Widom line, emanating from the liquid–liquid critical point (LLCP). Although these studies made a significant contribution to the current understanding of the above phenomena, a detailed molecular picture is still lacking. Recently, our group has explained the diffusion–viscosity decoupling from a jump-diffusion perspective. The jump-diffusion coefficient, emanating from the jump translation of water molecules, is calculated using a quantitative approach for different temperatures at ambient pressure. It has been observed that jump-diffusion is the key factor for diffusion–viscosity decoupling in supercooled water. The same method is adopted in the present work to estimate the jump-diffusion coefficient for different T–P state points and, thereby, explains the role of jump-diffusion for the different extents of the SE relation breakdown at different pressures. The residual diffusion coefficient, the other component of the total diffusion that originates from small step displacement and that is calculated by subtracting the jump-diffusion coefficient from the total diffusion, is seen to be fairly coupled to the viscosity at the entire range of temperature and pressure. Furthermore, we have calculated the average number of H-bonds per water molecule and the tetrahedral order for different T–P state points and investigated an approximate correlation between the average local structure and the contribution of the jump-diffusion to the total diffusion of water. This study, therefore, puts forward a new perspective for explaining the SE relation breakdown in supercooled water under different pressure conditions.

Although water is the most ubiquitous liquid it shows many thermodynamic and dynamic anomalies. Some of the anomalies further intensify on cooling down below the freezing point and thereby reaching...

KdpFABC is an ATP-dependent K+ pump that ensures bacterial survival in K+-deficient environments. Whereas transcriptional activation of kdpFABC expression is well studied, a mechanism for down regulation when K+ levels are restored has not been described. Here we show that KdpFABC is inhibited when cells return to a K+-rich environment. The mechanism of inhibition involves phosphorylation of Ser162 on KdpB, which can be reversed in vitro by treatment with serine phosphatase. Mutating Ser162 to Alanine produces constitutive activity, whereas the phosphomimetic Ser162Asp mutation inactivates the pump. Analyses of the transport cycle show that serine phosphorylation abolishes the K+-dependence of ATP hydrolysis and blocks the catalytic cycle after formation of the aspartyl phosphate intermediate (E1~P). This regulatory mechanism is unique amongst P-type pumps and this study furthers our understanding of how bacteria control potassium homeostasis to maintain cell volume and osmotic potential.

A recent experiment has directly checked the validity of the Stokes−Einstein (SE) relation for pure water, pure methanol, and their binary mixtures of three different compositions at different temperatures. The effect of composition on the nature of breakdown of the SE relation is interesting. While in the majority of the systems, an increasing SE breakdown is observed with decreasing temperature, the breakdown is already significant at higher temperatures for the equimolar mixture. Violations of the SE relation in pure supercooled water at different temperatures and pressures have been previously explained using the translational jump-diffusion (TJD) approach, which provides a fundamental molecular basis, by directly connecting the SE breakdown with jump-diffusion of the molecules. We have used the same TJD approach for explaining the SE breakdown for the methanol/water binary mixtures of compositions studied in the experiment over a wide range of temperatures between 220 K and 300 K. We have understood that the jump-diffusion is the key responsible factor for the SE breakdown. The maximum jump-diffusion contribution gives rise to the early SE breakdown observed for the equimolar mixture observed in the experiment. This study, therefore, provides molecular insight into the SE breakdown for the supercooled water/methanol binary mixture, as found in the experiment.