Liposomes and Lipid Droplets Display a Reversal of Charge-Induced Hydration Asymmetry.

The unique properties of water are critical for life. Water molecules have been reported to hydrate cations and anions asymmetrically in bulk water, being a key element in the balance of biochemical interactions. We show here that this behavior extends to charged lipid nanoscale interfaces. Charge hydration asymmetry was investigated by using nonlinear light scattering methods on lipid nanodroplets and liposomes. Nanodroplets covered with negatively charged lipids induce strong water ordering, while droplets covered with positively charged lipids induce negligible water ordering. Surprisingly, this charge-induced hydration asymmetry is reversed around liposomes. This opposite behavior in charge hydration asymmetry is caused by a delicate balance of electrostatic and hydrogen-bonding interactions. These findings highlight the importance of not only the charge state but also the specific distribution of neutral and charged lipids in cellular membranes.

Three-Dimensional Confinement of Water: H2O Exhibits Long-Range (>50 nm) Structure while D2O Does Not.

Water is the liquid of life thanks to its three-dimensional adaptive hydrogen (H)-bond network. Confinement of this network may lead to dramatic structural changes influencing chemical and physical transformations. Although confinement effects occur on a <1 nm length scale, the upper length scale limit is unknown. Here, we investigate 3D-confinement over lengths scales ranging from 58-140 nm. By confining water in zwitterionic liposomes of different sizes and measuring the change in H-bond network conformation using second harmonic scattering (SHS), we determined long-range confinement effects in light and heavy water. D2O displays no detectable 3D-confinement effects <58 nm (<3 × 106 D2O molecules). H2O is distinctly different. The vesicle enclosed inner H-bond network has a different conformation compared to the outside network and the SHS response scales with the volume of the confining space. H2O displays confinement effects over distances >100 nm (>2 × 107 H2O molecules).

Temperature dependence of water-water and ion-water correlations in bulk water and electrolyte solutions probed by femtosecond elastic second harmonic scattering.

The temperature dependence of the femtosecond elastic second harmonic scattering (fs-ESHS) response of bulk light and heavy water and their electrolyte solutions is presented. We observe clear temperature dependent changes in the hydrogen (H)-bond network of water that show a decrease in the orientational order of water with increasing temperature. Although D2O has a more structured H-bond network (giving rise to more fs-ESHS intensity), the relative temperature dependence is larger in H2O. The changes are interpreted in terms of the symmetry of H-bonds and are indicators of nuclear quantum effects. Increasing the temperature in electrolyte solutions decreases the influence of the total electrostatic field from ions on the water-water correlations, as expected from Debye-Hückel theory, since the Debye length becomes longer. The effects are, however, 1.9 times (6.3 times) larger than those predicted for H2O (D2O). Since fs-ESHS responses can be computed from known molecular coordinates, our observations provide a unique opportunity to refine quantum mechanical models of water.

Charge Gradients around Dendritic Voids Cause Nanoscale Inhomogeneities in Liquid Water.

Water is the matrix of life and serves as a solvent for numerous physical and chemical processes. The origins of the nature of inhomogeneities that exist in liquid water and the time scales over which they occur remains an open question. Here, we report femtosecond elastic second harmonic scattering (fs-ESHS) of liquid water in comparison to an isotropic liquid (CCl4) and show that water is indeed a nonuniform liquid. The coherent fs-ESHS intensity was interpreted, using molecular dynamics simulations, as arising from charge density fluctuations with enhanced nanoscale polarizabilities around transient voids having an average lifetime of 300 fs. Although voids were also present in CCl4, they were not characterized by hydrogen bond defects and did not show strong polarizability fluctuations, leading to fs-ESHS of an isotropic liquid. The voids increased in number at higher temperatures above room temperature, in agreement with the fs-ESHS results.