THE ORGANISATION OF CONFINED WATER IN SELF-ASSEMBLED LIPID NANOSTRUCTURES

Phospholipid-based liposomes are abundantly studied in biomembrane research and used in numerous medical and biotechnological applications. When dispersed in water, lipids hydrate to form a variety of complex nanostructures. Despite our knowledge of 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, we have investigated the nature of the confined water layer for variety of lipids, focusing on the phosphatidylcholine (PC) phosphatidylethanolamine (PE) species. The majority of the studies are conducted in the fluid lamellar phase of multilamellar vesicles with and without the inclusion of ions dissolved in the water phase. Additionally, a binary lipid mixture in the inverse hexagonal (HII) phase was also investigated. We are proposing a new model for describing three different water regions, which have been characterised, using a combination of Small Angle X-ray Scattering (SAXS) and volumetric data. 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, influences of chain saturation, headgroup type and as a function of ion concentration, influenced by mono- and divalent ions. Under temperature, the overall water layer and perturbed water layer thickness increase, whilst the free water layer displays the opposite trend for PCs and, remarkably, in PEs the free water layer is completely absent. This behaviour in PEs is different when in the inverse hexagonal phase, where a free water region develops and remains relatively unchanged as the micelle packing frustration is alleviated. Most interestingly, the mechanical behaviour of the inverse hexagonal phase is different orientated towards the corners and flat sides of the Wigner Seitz cell. Understandably, the disorder is relatively enhanced within the hexagon’s corners (decompression zones), concurrently with the amount of perturbed waters in this region. The influence of ions onto the lamellar phase water layers is heavily dependent on ion valency and also on a specific ions kosmotropic or chaotropic potential. The trend of the cationic Hofmeister series is reflected in the perturbed water layer, increasing from Mg2+, Na+ to K+.