Understanding the Dehydration Stress in Lipid Vesicles by a Combined Quartz Crystal Microbalance and Dielectric Spectroscopy Study

Abstract

Herein, 1‐palmitoyl‐2‐oleoyl‐glycero‐3‐phosphocholine (POPC) vesicles are studied using a newly designed setup that combines dielectric and gravimetric (quartz crystal microbalance, QCM) measurements. This setup allows to monitor the molecular dynamics and phase transitions of lipid structures under controlled humidity and temperature conditions. The main relaxation process (R2), related to the intrinsic headgroups' dynamics, is found in all phases and reveals a systematic acceleration with increasing hydration level. In contrast, a faster relaxation mode (R1) is found exclusively in the “liquid‐crystalline,” high‐temperature state, favoring water clusters located in the disordered aliphatic phase as origin. A third process (R3), being slower than the R2 mode, is attributed to a decoupled dynamics of hydration water in the hydrophilic part of the lipid bilayers occurring at higher hydration levels. In addition to molecular dynamics, both QCM and dielectric relaxation spectroscopy (DRS) measurements are demonstrated to yield consistent information on the phase behavior of the POPC samples which show a marked sensitivity to the hydration level. Moreover, contact angle measurements and atomic force microscopy confirm irreversible changes from the initial vesicles to a stacked bilayer structure upon repeated hydration‐dehydration, which manifests in the hydrophobization of the surface and water reduced absorption kinetics.

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

“…Altogether, the observed slowing down of the diffusion with an increase in diffusion activation energy suggests that the SLB quasi-gellifies (stiffens) upon dehydration, in particular upon the removal of the clathrate water molecules. This is in agreement with previous studies, which indeed suggested that dehydration leads to an increase in the main phase transition temperature of lipids 63,64 indicating fluid-to-gel like transition at lower hydration conditions.…”

Hydration Layer of Only a Few Molecules Controls Lipid Mobility in Biomimetic Membranes

“…Altogether, the observed slowing down of the diffusion with an increase in diffusion activation energy suggests that the SLB quasi-gellifies (stiffens) upon dehydration, in particular upon the removal of the clathrate water molecules. This is in agreement with previous studies, which indeed suggested that dehydration leads to an increase in the main phase transition temperature of lipids, 79,80 indicating fluid-to-gel-like transition at lower hydration conditions.…”