Location of sugars in bilayer and non-bilayer lipid phases: Relevance to membrane preservation during desiccation
Creators: name: Kent, Benjamin
Publisher: name: RMIT University
Year: 2024

description: The removal of liquid water from a lipid membrane system, whether through desiccation or slow freezing, is known to cause irreparable damage to biological organisms. A significant factor in this damage is the occurrence of lipid phase transitions in cell membranes. These transitions, induced by changes in hydration or temperature, can involve changes in the bilayer structure of the lipids (bilayer-bilayer transition), or a rearrangement of the lipids into an inverse phase (bilayer-non bilayer transition). In both cases, the semi-permeability of the fluid lipid bilayer membrane – critical to normal cell function – is lost, and in the case of bilayer-non bilayer transitions, the membrane structure is irreversibly damaged.<br><br>Sugars have been shown to play an important role in the natural defences of biological organisms by altering the dehydration induced phase behaviour of lipid membranes. Their accumulation in lipid membrane systems helps to preserve the fluid lipid bilayer phase at low hydration by preventing, or delaying the onset of, deleterious lipid phase transitions. However, while this attribute of sugars is well known, there is still debate surrounding the mechanisms by which sugars stabilise membranes. Central to this debate is the nature of the sugar-lipid interaction during dehydration and freezing events. <br><br>This thesis presents the results of investigations into the effects of sugars on the phase behaviour and structure of DOPC/DOPE phospholipid mixtures. Small angle x-ray scattering and Differential Scanning Calorimetry were employed to determine the phase behaviour of these systems at a range of osmotic pressures in the temperature range 20 °C – 80 °C. The transition from the inverse hexagonal phase to the fluid inverse ribbon phase is thoroughly characterised for the first time. This transition is continuous, with very low enthalpy. The presence of sugar completely inhibits the formation of the ribbon phase in favour of the inverse hexagonal phase. <br><br>The glucose – DOPE interaction is investigated at full hydration using contrast variation small angle neutron scattering. Glucose is partially excluded from the inverse hexagonal phase into a coexisting excess water microphase. The results indicate a preference for the lipid headgroups to associate with water, leading to the formation of a hydration layer of water adjacent to the lipid boundary. Measurements of the gel bilayer phase at -15 °C show qualitatively different sugar partitioning, with ice formation excluding sugar from the excess water microphase. A corresponding increase in the sugar concentration in the water layer between lipid bilayers is observed at this temperature. Membrane diffraction of substrate supported aligned DOPC bilayers was used to provide high resolution information on the location of sugars between fluid lipid bilayers. In agreement with the sugar partitioning results, these results show that the sugars are located, on average, near the centre of the water layer between lipid bilayers, with no evidence of direct close range interactions between sugars and lipids.<br>