Abstract
Water is essential to life and its translational motion in living systems mediates various biological processes, including transportation of function-required ingredients and facilitating the interaction between biomacromolecules. By combining neutron scattering and isotopic labeling, the present work characterizes translational motion of water on a biomolecular surface, in a range of systems: a hydrated protein powder, a concentrated protein solution, and in living Escherichia coli (E. coli) cells. Anomalous sub-diffusion of water is observed in all samples, which is alleviated upon increasing the water content. Complementary molecular dynamics simulations and coarse-grained numerical modeling demonstrated that the sub-diffusive behavior results from the heterogeneous distribution of microscopic translational mobility of interfacial water. Moreover, by comparing the experimental results measured on E. coli cells with those from a concentrated protein solution with the same amount of water, we show that water in the two samples has a similar average mobility, however the underlying distribution of motion is more heterogeneous in the living cell.
Highlights
Water is an active ingredient in cell biology
By comparing the experimental results measured on E. coli cells with those from a concentrated protein solution with the same amount of water, we show that water in the two samples has a similar average mobility, the underlying distribution of motion is more heterogeneous in the living cell
To examine the water dynamics on a protein surface, we conducted neutron scattering experiments on H2O-hydrated perdeuterated Cytochrome P450 protein (CYP) at 280 K at four hydration levels, i.e., h 1⁄4 0.4, 1.0, 2.0, and 4.0. h 1⁄4 0.4 correspond to a case that the protein surface is covered roughly by a single layer of water molecules,36 while h 1⁄4 4.0 denotes multilayers of surface water,37,38 corresponding to a concentrated solution
Summary
Water is an active ingredient in cell biology. Biomacromolecules inside the cell are encapsulated in a shell of hydration water, whose structure and dynamics are distinct from pristine bulk water. This hydration shell of water plays a key role in stabilizing the 3-D structure of the biomolecules and lubricating them to exhibit the required flexibility for function. the diffusive motions of water aid ligand and proton transfer, protein–DNA and protein–ligand recognition, protein dynamical transition, and folding of the protein molecule into the correct 3-D structure.. As neutrons are highly sensitive to hydrogen atoms, whose incoherent scattering cross section is an order of magnitude larger than incoherent/coherent scattering cross section of other elements, one can perform neutron scattering on perdeuterated biosystems hydrated in H2O to selectively study the motions of water.20–27 This method has been applied to explore the dynamics of water in living cells, which was shown to be highly retarded as compared to bulk water.. A systematic investigation on how this anomalous diffusion of water evolves from a hydrated protein powder, to a protein solution, to a living cell is lacking, and the microscopic mechanism governing the evolution is unknown To this end, we present here results from neutron scattering experiments on perdeuterated proteins hydrated by H2O at a series of well-controlled hydration levels, to investigate the translational mobility of water molecules involved. By comparing the experimental results measured on E. coli cells with those from a concentrated protein solution with the same amount of water, we found that the distribution of water mobility in the living cells is much broader
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