Water Dynamics in Bulk and at Biological Interfaces

Abstract: Abstract in Undetermined Despite being one of the most well studied compounds on earth, with tremendous implication for biology, chemistry and industrial applications, the complete molecular picture of water still eludes the modern scientific community. In this work we address the especially controversial question about water dynamics. We investigate water rotational and translational dynamics in bulk water as well as close to biologically relevant surfaces; small organic molecules, amino acids and globular protein macromolecules and we also investigate the temperature dependence of this dynamics. To further extend the experimental temperature window we supercool our samples down to ~235 K, either by emulsification or by the use of thin capillaries. The major experimental technique used was Nuclear Magnetic Resonance spin relaxation measurements which reports directly on the rotational correlation time of a molecule and has been used earlier with great success to study the hydration of proteins and cells. We have also performed Neutron Scattering experiment and Molecular Dynamics simulations to complement our spin relaxation measurements. For bulk water we show that supercooled water translates and rotates in jumps rather than through a continuous diffusion. For small solutes we show that the dynamical perturbation is modest, less than a factor of 2 at room temperature. We further show that the temperature dependence of this perturbation is non-monotonic, going through a maximum around ~255 K and then decreasing at lower temperature. Our experiments also indicate that the dynamical perturbation is larger from hydrophobic solutes. For proteins we show that the dynamical perturbation from the surface water is dominated by a small number of water molecules which presumably resides in concavities and surface pockets. Except for these protein specific water molecules the large number of remaining hydration water is much less perturbed, approximately a factor of 2 compared to bulk water. Hence we find no quantitative or qualitative difference between the majority of water molecules hydrating a protein surface and water molecules hydrating small organic molecules and amino acids. We also extend and improve the existing model and methodology used to interpret Quasi Elastic Neutron Scattering experiments on aqueous solutions as the model most often used introduces artifacts in the interpretation.