Cooperativity between sodium ions and water molecules facilitates lipid mobility in model cell membranes

Abstract

Cellular membranes are surrounded by an aqueous buffer solution containing various ions, which influence the hydration layer of the lipid head groups. At the same time, water molecules hydrating the lipids play a major role in facilitating the organisation and dynamics of membrane lipids. Employing fluorescence microscopy imaging and fluorescence recovery after photobleaching measurements, we demonstrate that the cooperativity between water and sodium (Na+) ions is crucial to maintain lipid mobility upon the removal of the outer hydration layer of the lipid membrane. At similar hydration conditions, lipid diffusion ceases in absence of Na+ ions. We unravel that Na+ ions strengthen the water clathrate cage around the lipid phosphocholine head group and thus prevent its breaking upon removal of bulk water. Intriguingly, divalent cation Ca2+ does not show this effect. In this article we provide a detailed molecular-level picture of ion specific dependence of lipid mobility and membrane hydration properties.

Nanoscale structural response of biomimetic cell membranes to controlled dehydration

“…One way of describing membrane dehydration would be to define a state where the amount of hydrating water decreases to a level where the dynamic or structural properties of the membrane begin to be affected. Based on the experiments reported here as well as our recent works, 37,50,51 both the dynamics and the structure of the membrane are gradually affected when the first hydration shell of the lipids is perturbed. In the case of the zwitterionic phosphocholine lipids, this happens when the hydration drops below about 12 molecules per lipid.…”

“…The observed changes in lipid partitioning are associated with the decreased hydrophobic mismatch, but unlike the height mismatch data, they are also strongly dependent on the lipid mobility. Since lipid mobility decreases strongly with decreasing membrane hydration, 37,50,51 it is not too surprising that we do not see a steady increase in the partitioning of the L d phase lipids into the L o phase. At lower membrane hydration states, we observed a competition between the lower hydrophobic mismatch, which favours lipid admixing, and the lower diffusivity, which hinders mixing.…”

“…In addition, other recent studies of ours have shown that the dynamics of lipids under dehydration conditions is strongly influenced by the ionic composition of the buffer hydrating the membrane. 51 Although lipids remain mobile in the presence of Na + ions at humidity above 50% RH, their mobility is significantly reduced in MilliQ water upon dehydration to 85% RH. In the course of the measurements presented here, we decided to perform AFM scanning in the absence of salt-enriched buffer, which could lead to salt crystallisation on the membranes.…”

Nanoscale structural response of biomimetic cell membranes to controlled dehydration

“…At 50% RH and less, the mobility of lipids is almost completely ceased, which results from the breaking of the first hydration shell surrounding a single lipid head group moiety. Furthermore, other recent studies of ours showed that the dynamics of lipids under dehydration conditions is strongly affected by the ionic composition of the buffer hydrating the membrane 50 . Although lipids in the presence of Na + ions remain mobile in humidity above 50% RH, in MilliQ water upon dehydration to 85% RH their mobility is already significantly reduced.…”