Planar Confined Water Organisation in Lipid Bilayer Stacks of Phosphatidylcholine and Phosphatidylethanolamine

Abstract

Phospholipid-based liposomes are abundantly studied in biomembrane research and used in numerous medical and biotechnological applications. Despite current extensive knowledge on membrane nanostructure and its mechanical properties under various environmental conditions, there is still a lack of understanding on interfacial lipid-water interactions. In this work, the nature of the confined water layer for L-α-phosphatidylcholine (egg-PC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dimyristoyl-sn-glycerol-3-phosphatidyl-choline (DMPC) and 1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine (DMPE) in the fluid lamellar phase of multilamellar vesicles was investigated. A new model for describing three different water regions is proposed, which have been characterised using a combination of small angle X-ray scattering (SAXS) and densitometry. The three regions concern (i) ‘the headgroup water’, (ii) ‘perturbed water’ near the membrane interface and (iii) a core layer of ‘free water’ (unperturbed water). The behaviour of all three layers is discussed as a function of temperature, concerning influences of chain saturation and headgroup type. While the overall water layer and perturbed water layer thickness increase with temperature, the free water layer displays the opposite trend for PCs, and in PEs is completely absent. Furthermore, an estimate of the temperature dependent headgroup orientation is given for both, PCs and PEs. The newly presented structural data deduced from the three-water region model will be useful for future refined molecular dynamics simulations and allow a better theoretical understanding of the attractive van der Waals force between adjacent membranes.

Version notes

Added the funder EPSRC also to the MS and added the missing author Amin Sadeghpour to the ESI.

Content

Supplementary material

Electronic supporting information (ESI) of "Planar Confined Water Organisation in Lipid Bilayer Stacks of Phosphatidylcholine and Phosphatidylethanolamine"
Content - Figure S1: Examples of global fitting of SAXS data of DMPC, egg-PC and DOPC. - Table S1: Lattice spacings and form factor values of DMPE. - Derivation of the parameter DH2. - Table S2: Structural parameters of DMPC, egg-PC and DOPC (30 °C) and DMPE (60 °C)