DMSO Induces Dehydration near Lipid Membrane Surfaces

Abstract

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.

Correlating steric hydration forces with water dynamics through surface force and diffusion NMR measurements in a lipid–DMSO–H 2 O system

“…We see now that the monotonically increasing vesicle surface water diffusivity in going from 0 mol% to 7.5 mol% DMSO, as measured in ref. 20, coincides with a decreased range of repulsive forces and subsequently greater adhesion. DMSO weakens water binding to the head groups, which causes the interfacial water to diffuse faster (more like water diffuses in the bulk) and a shortened range of the repulsive force through a decrease in hydrated excluded volume.…”

“…The first regime, illustrated in Fig. 3B (C DMSO = 2-7.5 mol%), is partly characterized by unchanging head group ordering and dynamics according to 31 P NMR anisotropy measurements and the electron paramagnetic resonance (EPR) line shape of nitroxide spin probes tethered to the PC group (20). Scattering studies in the literature, although relatively insensitive to choline position, have also shown no indication of a change in head group conformation over this concentration range (14).…”

“…3C (C DMSO = 10-20 mol%), head groups are gradually collapsed down onto the surface, and their mobility suppressed, which is, once again, corroborated by the EPR and NMR measurements in ref. 20. The repulsive force in Fig.…”

“…dioleoylphosphatidylcholine (DOPC, fluid phase) vesicles using the Overhauser Dynamic Nuclear Polarization relaxometry technique (19) show a continual increase in water diffusivity with DMSO concentration (20). Water diffusivity, D w , near the head groups increased ∼50% in going from 0 mol% DMSO (pure water; D w = 7.7 × 10 −10 m 2 /s) to 7.5 mol% DMSO (D w = 11.4 × 10 −10 m 2 /s) for DPPC membranes, whereas the mobility and ordering of PC head groups remained constant up to 10 mol% (20).…”

“…Water diffusivity, D w , near the head groups increased ∼50% in going from 0 mol% DMSO (pure water; D w = 7.7 × 10 −10 m 2 /s) to 7.5 mol% DMSO (D w = 11.4 × 10 −10 m 2 /s) for DPPC membranes, whereas the mobility and ordering of PC head groups remained constant up to 10 mol% (20). This hydration dynamics study is perhaps the strongest evidence yet of DMSO weakening the cohesion of the surface water, which suggests that head group dehydration is facilitated at DMSO concentrations <10 mol%.…”