Hydration layer of only few molecules controls lipid mobility in biomimetic membranes

Abstract

Self-assembly of biomembranes results from the intricate interactions between water and the lipids' hydrophilic head groups. Therefore, the lipid-water interplay strongly contributes to modulating membranes architecture, lipid diffusion, and chemical activity. Here, we introduce a new method of obtaining dehydrated, phase-separated, supported lipid bilayers (SLBs) solely by controlling the decrease of their environment's relative humidity. This facilitates the study of the structure and dynamics of SLBs over a wide range of hydration states. We show that the lipid domain structure of phase-separated SLBs is largely insensitive to the presence of the hydration layer. In stark contrast, lipid mobility is drastically affected by dehydration, showing a 6-fold decrease in lateral diffusion. At the same time, the diffusion activation energy increases approximately twofold for the dehydrated membrane. The obtained results, correlated with the hydration structure of a lipid molecule, revealed that about 6-7 water molecules directly hydrating the phosphocholine moiety play a pivotal role in modulating lipid diffusion. These findings could provide deeper insights into the fundamental reactions where local dehydration occurs, for instance during cell-cell fusion, and help us better understand the survivability of anhydrobiotic organisms. Finally, the strong dependence of lipid mobility on the number of hydrating water molecules opens up an application potential for SLBs as very precise, nanoscale hydration sensors.

Laurdan discerns lipid membrane hydration and cholesterol content

“…Therefore, we reason that the peculiar dehydration-induced behavior of L d phase in phase-separated membrane might be due to redistribution of components between L o and L d domains, most likely involving cholesterol. This would rationalize also our previous findings, 1 that with reduced hydration, the hydrophobic mismatch between the L d and L o domains decreases significantly (cholesterol influx to L d phase would increase its thickness, thus lessening the hydrophobic mismatch between domains). To test our hypothesis, we conducted the fluorescence microscopy experiments with fluorescently labeled cholesterol, the results of which are depicted in Figure S7.…”

“…Comprehensive data on this issue can be found in our previous work. 1 Analysis of the GP parameter as a function of hydration level of Ld domains reveals interesting behavior (Fig 3a, green part). In the range from bulk hydration to 50% RH, a gradual, small increase in GP can be observed, after which it remains constant down to 20% RH, and then it increases slightly again.…”

“…32 It has been shown that upon removal of bulk water and exposing the lipid bilayers to 95% RH, the first solvation shell is largely preserved and only further reduction of hydration degree breaks it down. 1 However, even extreme dehydration does not remove water strongly bound to lipids, particularly these associated with the carbonyls. 31 Having this molecular picture in mind, we infer that upon membrane dehydration, the most drastic changes in hydration occur in the outer regions of the phospholipid bilayer, while the number of water molecules in the vicinity of Laurdan fluorophore barely changes.…”

“…Membrane hydration state was varied by applying a drying process with a slow and sequential reduction in the sample environment’s relative humidity (RH). 1…”

“…Furthermore, in addition to the slowdown of interfacial water dynamics, both the structural and dynamical properties of lipid bilayers have been found to be affected by water content. 1,[35][36][37] Membrane dehydration results in a decrease in the area and volume per lipid and a concomitant increase in membrane thickness, as well as a slowdown in the lipid translational and rotational mobility, ultimately leading to a liquid-disordered to gel phase transition. The results of the log-normal decomposition reinforce the idea that during membrane dehydration there is no significant change in the hydration level at the Laurdan site.…”