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.

From angular to round: in depth interfacial analysis of binary phosphatidylethanolamine mixtures in the inverse hexagonal phase

“…Note, each diffraction peak was fitted individually and the resulting Z values were averaged to retrieve one s value referring to the h1 0i direction. Following the recently published three-water layer model, 52 and having estimated the membrane-fluctuation distance, s, allows us to divide the water core into three distinct regions: (i) the ''headgroup region'' associated with the water volume around the headgroup, (ii) the ''perturbed region'' defined by the fluctuation distance s, and (iii), the ''free water region'', which is unperturbed by the fluctuating lipid monolayer. For the calculation of the total water volume, we used the approximation: 32…”

“…52 The same accounts for number of perturbed waters per lipid (about 3 waters compare to 6 waters in DMPE at 80 1C 52 ). The latter differences are explained by the smaller area per lipid in the inverse hexagonal phase (A p = 0.45 nm 2 in the H II -phase compares to A p = 0.64 nm 2 in DMPE at 80 1C 52 ) and its molecular wedge shape (Fig. 1B).…”