Communication: Contrasting effects of glycerol and DMSO on lipid membrane surface hydration dynamics and forces

Abstract

Glycerol and dimethyl sulfoxide (DMSO) are commonly used cryoprotectants in cellular systems, but due to the challenges of measuring the properties of surface-bound solvent, fundamental questions remain regarding the concentration, interactions, and conformation of these solutes at lipid membrane surfaces. We measured the surface water diffusivity at gel-phase dipalmitoylphosphatidylcholine (DPPC) bilayer surfaces in aqueous solutions containing ≤7.5 mol. % of DMSO or glycerol using Overhauser dynamic nuclear polarization. We found that glycerol similarly affects the diffusivity of water near the bilayer surface and that in the bulk solution (within 20%), while DMSO substantially increases the diffusivity of surface water relative to bulk water. We compare these measurements of water dynamics with those of equilibrium forces between DPPC bilayers in the same solvent mixtures. DMSO greatly decreases the range and magnitude of the repulsive forces between the bilayers, whereas glycerol increases it. We propose that the differences in hydrogen bonding capability of the two solutes leads DMSO to dehydrate the lipid head groups, while glycerol affects surface hydration only as much as it affects the bulk water properties. The results suggest that the mechanism of the two most common cryoprotectants must be fundamentally different: in the case of DMSO by decoupling the solvent from the lipid surface, and in the case of glycerol by altering the hydrogen bond structure and intermolecular cohesion of the global solvent, as manifested by increased solvent viscosity.

FIG. 1.

(a) Diffusivity of water in the bulk, D_bulk, in the absence of lipid vesicles, (b) water diffusivity, D_surface, at the surface of DPPC large unilamellar vesicles, and (c) the ratio of D_surface and D_bulk at various molar fractions of DMSO (X_DMSO) and glycerol (X_glycerol) at 25 °C. The error bars represent the standard deviation of the parameters estimated from the fitting.

FIG. 2.

Forces, F, measured with the SFA between mica-supported gel-phase DPPC bilayers in DMSO–water and glycerol–water mixtures at 22 °C and pH 6.0 ± 0.2. Solid, colored lines were drawn by hand. (a) Static measurements of repulsive forces on a semilog plot. (b) Adhesion forces measured upon slow separation of the surfaces. Measurements were made at X_DMSO and X_glycerol values of 0, 0.02, 0.05, and 0.075, and the trends were consistent, but due to overlap between force curves, only certain data series are shown here for clarity.