Phase behaviour, reorganization, and dynamics of lipid and soft matter assemblies

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One defining aspect of soft and biological materials is the extent to which they respond to various perturbing influences. The three complementary objectives of my proposed program aim at gaining a better understanding of how self-assembled soft materials, particularly lipid assemblies and deformable colloids, reorganize in response to changes in pressure or temperature or to molecular perturbations. My group uses solid-state nuclear magnetic resonance (NMR) to characterize soft materials via motions of their component molecules.Lipids molecules have water-soluble heads and oily chains. In water they self-assemble into fascinating and important structures like the bilayers underlying cell membranes. Processes that reorganize lipid assemblies are important both in biology and in soft materials science. Examples include membrane fusion, the spreading of a surfactant layer at air-water interfaces in lungs, and bacterial membrane disruption by antimicrobial polypeptides (AMPs) with specific structural properties. Synthetic examples include “bicellar” mixtures of lipids in which separation of lipids with mismatched chain-lengths results in small particles that progressively coalesce into more extended structures on heating. Studying soft materials at different pressures can separate effects of temperature and molecular packing. One program objective is to understand how interactions between molecules with different properties determine lipid assembly organization by observing how pressure affects bilayers. By studying lipid-cholesterol mixtures at different pressures, we will compare effects, on mixture properties, of interactions at the bilayer surface and in its interior. By studying bicellar dispersions at high pressure, we will explore how novel phases, like chain-interdigitated gel, result from a mismatch of lipid properties. These studies will employ a unique variable-pressure deuterium NMR probe.Our second objective is to better understand how polypeptides can drive lipid assembly reorganization. With a collaborator in Biochemistry, we will use NMR to directly observe effects of antimicrobial peptides (AMPs) on bacterial membranes with deuterium-labeled components. By controlling pH, we will also study how the membrane-perturbing capacities and orientations of novel AMPs, from cod, change with peptide charge. In lungs, the essential surfactant function depends on small proteins that help transfer material, via appropriately structured bridges, between lipid layers. We will study such processes in model systems by observing how specific peptides, derived from lung surfactant, affect the merging of bilayered micelles into larger structures. Microgels are deformable cross-linked polymer colloids with rich behaviours not fully understood at the molecular level. Microgels that undergo a collapse transition, along with a large-scale release of water, have many potential applications including drug delivery. Our third objective is to use variable-pressure NMR, in collaboration with colleagues at Memorial and at Lund University, Sweden, to study polymer motions in such materials and to characterize local structure through the collapse transition. Along with enhancing our understanding of processes that can reorganize soft materials, this program will help advance technologies like improved artificial lung surfactant therapies for respiratory problems, AMPs to mitigate acquired antibiotic resistance, and drug delivery strategies (liposome encapsulation, colloidal trapping). Novel aspects include the use of bilayered micelle coalescence to model biological bilayer reconstruction, the insertion of deuterium labels into colloidal and bacterial systems, and the use of variable-pressure NMR for soft matter.