Jinsuk Song

DMSO Induces Dehydration near Lipid Membrane Surfaces

Dimethyl sulfoxide (DMSO) has been broadly used in biology as a cosolvent, a cryoprotectant, and an enhancer of membrane permeability, leading to the general assumption that DMSO-induced structural changes in cell membranes and their hydration water play important functional roles. Although the effects of DMSO on the membrane structure and the headgroup dehydration have been extensively studied, the mechanism by which DMSO invokes its effect on lipid membranes and the direct role of water in this process are unresolved. By directly probing the translational water diffusivity near unconfined lipid vesicle surfaces, the lipid headgroup mobility, and the repeat distances in multilamellar vesicles, we found that DMSO exclusively weakens the surface water network near the lipid membrane at a bulk DMSO mole fraction (XDMSO) of <0.1, regardless of the lipid composition and the lipid phase. Specifically, DMSO was found to effectively destabilize the hydration water structure at the lipid membrane surface at XDMSO <0.1, lower the energetic barrier to dehydrate this surface water, whose displacement otherwise requires a higher activation energy, consequently yielding compressed interbilayer distances in multilamellar vesicles at equilibrium with unaltered bilayer thicknesses. At XDMSO >0.1, DMSO enters the lipid interface and restricts the lipid headgroup motion. We postulate that DMSO acts as an efficient cryoprotectant even at low concentrations by exclusively disrupting the water network near the lipid membrane surface, weakening the cohesion between water and adhesion of water to the lipid headgroups, and so mitigating the stress induced by the volume change of water during freeze-thaw.

Specific ions modulate diffusion dynamics of hydration water on lipid membrane surfaces.

Effects of specific ions on the local translational diffusion of water near large hydrophilic lipid vesicle surfaces were measured by Overhauser dynamic nuclear polarization (ODNP). ODNP relies on an unpaired electron spin-containing probe located at molecular or surface sites to report on the dynamics of water protons within ∼10 Å from the spin probe, which give rise to spectral densities for electron–proton cross-relaxation processes in the 10 GHz regime. This pushes nuclear magnetic resonance relaxometry to more than an order of magnitude higher frequencies than conventionally feasible, permitting the measurement of water moving with picosecond to subnanosecond correlation times. Diffusion of water within ∼10 Å of, i.e., up to ∼3 water layers around the spin probes located on hydrophilic lipid vesicle surfaces is ∼5 times retarded compared to the bulk water translational diffusion. This directly reflects on the activation barrier for surface water diffusion, i.e., how tightly water is bound to the hydrophilic surface and surrounding waters. We find this value to be modulated by the presence of specific ions in solution, with its order following the known Hofmeister series. While a molecular description of how ions affect the hydration structure at the hydrophilic surface remains to be answered, the finding that Hofmeister ions directly modulate the surface water diffusivity implies that the strength of the hydrogen bond network of surface hydration water is directly modulated on hydrophilic surfaces.

Nanometer-Scale Water- and Proton-Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes†

Nafion, the most widely used polymer for electrolyte membranes (PEMs) in fuel cells, consists of a fluorocarbon backbone and acidic groups that, upon hydration, swell to form percolated channels through which water and ions diffuse. Although the effects of the channel structures and the acidic groups on water/ion transport have been studied before, the surface chemistry or the spatially heterogeneous diffusivity across water channels has never been shown to directly influence water/ion transport. By the use of molecular spin probes that are selectively partitioned into heterogeneous regions of the PEM and Overhauser dynamic nuclear polarization relaxometry, this study reveals that both water and proton diffusivity are significantly faster near the fluorocarbon and the acidic groups lining the water channels than within the water channels. The concept that surface chemistry at the (sub)nanometer scale dictates water and proton diffusivity invokes a new design principle for PEMs.

Correlating steric hydration forces with water dynamics through surface force and diffusion NMR measurements in a lipid–DMSO–H2O system

Significance We use the common biological additive DMSO to show quantitatively the impact that surface-bound water has on interactions between lipid bilayers, the membranes that separate the interior of cells from the surroundings. We present a number of metrics to gauge the hydration of the bilayer surfaces and show how the metrics are affected by the concentration of DMSO in the solvent. This work further connects measurements of surface forces, surface structure and dynamics, and surface water diffusion with significant and broad implications for soft matter systems. Dimethyl sulfoxide (DMSO) is a common solvent and biological additive possessing well-known utility in cellular cryoprotection and lipid membrane permeabilization, but the governing mechanisms at membrane interfaces remain poorly understood. Many studies have focused on DMSO–lipid interactions and the subsequent effects on membrane-phase behavior, but explanations often rely on qualitative notions of DMSO-induced dehydration of lipid head groups. In this work, surface forces measurements between gel-phase dipalmitoylphosphatidylcholine membranes in DMSO–water mixtures quantify the hydration- and solvation-length scales with angstrom resolution as a function of DMSO concentration from 0 mol% to 20 mol%. DMSO causes a drastic decrease in the range of the steric hydration repulsion, leading to an increase in adhesion at a much-reduced intermembrane distance. Pulsed field gradient NMR of the phosphatidylcholine (PC) head group analogs, dimethyl phosphate and tetramethylammonium ions, shows that the ion hydrodynamic radius decreases with increasing DMSO concentration up to 10 mol% DMSO. The complementary measurements indicate that, at concentrations below 10 mol%, the primary effect of DMSO is to decrease the solvated volume of the PC head group and that, from 10 mol% to 20 mol%, DMSO acts to gradually collapse head groups down onto the surface and suppress their thermal motion. This work shows a connection between surface forces, head group conformation and dynamics, and surface water diffusion, with important implications for soft matter and colloidal systems.

Excess Charge Density and its Relationship with Surface Tension Increment at the Air—Electrolyte Solution Interface

The adsorption isotherms of probe cationic molecules were measured at various electrolyte solution interfaces by resonant second harmonic generation. The excess charge density was obtained by analyzing the isotherms; it increases with square root of the bulk electrolyte concentration. Its value is ion-specific and the amount of probe molecular adsorption follows the Hofmeister series. By calculating the pressure anisotropy at the interface, it is found that the ratio of surface tension increment to the bulk electrolyte concentration decreases with the square of the excess charge density. This is in good agreement with the experimental observations.

Second Harmonic Generation Study of Malachite Green Adsorption at the Interface between Air and an Electrolyte Solution: Observing the Effect of Excess Electrical Charge Density at the Interface

Understanding the differential adsorption of ions at the interface of an electrolyte solution is very important because it is closely related, not only to the fundamental aspects of biological systems, but also to many industrial applications. We have measured the excess interfacial negative charge density at air-electrolyte solution interfaces by using resonant second harmonic generation of oppositely charged probe molecules. The excess charge density increased with the square root of the bulk electrolyte concentration. A new adsorption model that includes the electrostatic interaction between adsorbed molecules is proposed to explain the measured adsorption isotherm, and it is in good agreement with the experimental results.

Mapping Out Protein Hydration Dynamics by Overhauser Dynamic Nuclear Polarization

Water molecules in the immediate vicinity of biomolecular surfaces (e.g., protein, lipid membranes, and DNA) are essential to mediate biological activities, such as enzyme activity, ligand binding, allosteric effect, or molecular recognition. For instance, the formation of a functional enzyme-substrate complex may be mediated by the retardation of water mobility at the active site of the enzyme. Hydrated water could also facilitate specific protein function, such as channel gating, whose kinetics has been found to be critically correlated with the rate of water fluctuations. Additionally, water is generally thought to be a catalyst for the hydrogen-bond rearrangements of a protein. The prevailing view is that water molecules actively contribute to the hydrophobic effect involved in protein-folding and ligand-binding events by modulating protein conformational changes through the formation or breaking of hydrogen bonds at protein–water interfaces. Interestingly, the diffusion dynamics of hydration water that entails hydrogen-bond rearrangement of the dynamic protein–water network may be critically coupled to protein dynamics, not only at the interfaces, but also at the core. A combination of the rapid hydrogen-bond rearrangements and the fast hydration dynamics at protein–water interfaces are suggested to be essential in increasing the protein structural flexibility and facilitating protein-ligand recognition.