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.

Figure 1

Ratio of water diffusion coefficient D_surface at membrane surface and bulk water diffusion coefficient D_bulk in MLVs (a) and LUVs (b) at various DMSO molar fraction (X_DMSO) at 25°C. (c) The changes in peak-to-peak linewidth ΔH_pp deducted from the EPR spectra of the TEMPO-PC attached on the surface of LUVs and MLVs at various X_DMSO at 25°C. ΔH_pp is inversely proportional to the rotational mobility of the nitroxide spin attached at the lipid choline. (d) ³¹P CSA (Δσ) of the phosphate group at LUV surfaces at various X_DMSO at 25°C. Yellow areas in (c) and (d) are the region of low DMSO concentration where ODNP is performed and presented in (a) and (b). The error bar represents standard deviation of the parameter estimated from the fitting. To see this figure in color, go online.

Figure 2

(a) DMSO concentration dependence of the repeat distance (d) of DPPC and DOPC MLVs measured by SAXS at 28°C. Yellow area is the DMSO concentration used in the ODNP experiment in Fig. 1, a and b. (b) Schematic diagram of MLV system. Repeat distance (d), bilayer thickness (d_pp), intermembrane distance (d_w), and surface area per lipid (A) are indicated in the figure. The literature values are cited from ( and 44). To see this figure in color, go online.

Figure 3

Activation energy (E_a) of water diffusion on the DPPC LUV surface in the presence and absence of DMSO. The dashed lines are a linear fit to the Arrhenius equation (E_a = 19.2 kJ/mol for bulk water).

Figure 4

Schematic of hypothesized DMSO orientations at DPPC LUV surfaces that represent a snapshot of the most proximal configuration between DMSO and the lipid headgroups in a dynamic equilibrium, where DMSO does not permanently replace the bound hydration water of the PC headgroups. The DMSO’s sulfur faces to lipid phosphate as represented (1), whereas the DMSO’s oxygen is mainly taken up by the nitrogen atom of choline as represented (2). Water molecules are neglected in the figure for simplicity. To see this figure in color, go online.