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

Abstract

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 solvationlength 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.dimethyl sulfoxide | hydration shell | membrane interactions | lipid solvation | hydrodynamic radius S olute additives (e.g., osmolytes or denaturants) often play a key role in modulating biological interactions, where molecules can recognize each other and form assemblies. Dimethyl sulfoxide [DMSO, (O = S)(CH 3 ) 2 ] and DMSO-water mixtures in particular have attracted particular interest in biology and chemistry. DMSO is one of the most commonly used cryoprotectants in cellular systems (1), where it colligatively reduces the melting point of aqueous solutions and exerts additional effects that prevent cellular damage on freezing/ vitrification. Additional effects not seen with conventional glycol-and saccharide-based cryoprotectants include an increase in lipid membrane permeability of an array of solutes (2), promotion of cell fusion (3) and differentiation (4), and enhanced membrane resealing after damage (5). These phenomena are well-documented along with their dependence on DMSO concentration, but the molecular behavior of DMSO near lipid membranes remains a mystery.Bulk properties of DMSO-water mixtures are well-characterized. The oxygen and sulfur atoms of DMSO have a partial negative and positive charge, respectively, giving rise to a dipole moment of 3.96 Debye that exceeds nearly all conventional solvents. Dielectric relaxation spectroscopy (6), molecular dynamics simulations (7), and 1 H NMR studies (8) have shown that bulk water forms hydrogen bonds...

Effects of dimethyl sulfoxide on surface water near phospholipid bilayers

“…made in the two reports [6,9]. The experimental studies using SFA and ODNP measurements [6,9] observed (i) a decrease of membrane repulsion [39] and (ii) enhanced local diffusivity of surface water in the presence of DMSO (X DMSO < 10 mol%), respectively.…”

“…The experimental studies using SFA and ODNP measurements [6,9] observed (i) a decrease of membrane repulsion [39] and (ii) enhanced local diffusivity of surface water in the presence of DMSO (X DMSO < 10 mol%), respectively. The authors surmised that the competition between DMSO and surface hydration water on the interaction with lipid head-groups led to weakening of the strength of the cohesive water network hydrating the membrane headgroups, and thus enhancing surface water diffusivity and dehydration of the bilayer surfaces.…”

“…While great progress has been made in understanding how DMSO affects the structural and dynamical properties of bulk water [5], the effects of DMSO on water molecules at lipid bilayer surfaces remain elusive. Surface forces apparatus (SFA) measurement between two supported DPPC bilayers [6] and the observed decreasing repeat distance of multi-lamellar structures with increasing X DMSO [7,8] suggest that DMSO dehydrates bilayer surfaces. Employing Overhauser dynamic nuclear polarization (ODNP) measurement that used the Tempo (2,2,6,6-tetramethylpiperidin-1-oxyl) moiety as a spin-label to probe the dynamics of water, Cheng [9] et al reported that water diffusion was enhanced in the vicinity of bilayer surfaces with increasing X DMSO .…”

“…Employing Overhauser dynamic nuclear polarization (ODNP) measurement that used the Tempo (2,2,6,6-tetramethylpiperidin-1-oxyl) moiety as a spin-label to probe the dynamics of water, Cheng [9] et al reported that water diffusion was enhanced in the vicinity of bilayer surfaces with increasing X DMSO . These companion studies [6,9] argue that both the surface dehydration and enhanced water diffusivity at the bilayer surfaces originate from DMSOweakened H-bonds of water with lipid head groups. Although the properties of surface water at bilayers is known to differ from those of bulk water [10,11], the DMSO enhanced-surface water diffusion is both intriguing and counter-intuitive, not easily reconciled with the general notion that DMSO slows down the water dynamics (e.g., diffusion, H-bond lifetime) (see Fig.S1) [5,12].…”

“…Although the properties of surface water at bilayers is known to differ from those of bulk water [10,11], the DMSO enhanced-surface water diffusion is both intriguing and counter-intuitive, not easily reconciled with the general notion that DMSO slows down the water dynamics (e.g., diffusion, H-bond lifetime) (see Fig.S1) [5,12]. Furthermore, the assumption that a strong interaction of DMSO with lipid head groups displaces hydration water and weakens the H-bond network near bilayer surfaces [7,13] was surmised based on a few pieces of indirect evidence stitched together from measurements conducted in bulk and force-distance profiles obtained by SFA [6], not based on a direct measurement probing the dynamics of water and DMSO on surfaces. We believe that this requires further study and confirmation.…”