Self-assembled biomimetic membranes and de-novo peptides for nanomedicine and biosensing

Liposome and polymersome are hollow bilayer vesicles nanostructures formed by self-assembly of phospholipids and synthetic amphiphilic block copolymers, respectively. Liposomes have been widely studied and developed for wide range of applications. On the other hand, polymersomes have several features which make them better suited than liposomes for certain technological applications, amongst which are their superior stability and excellent chemical versatility. Nevertheless, the self-assembly as well as the interaction of phospholipids and block copolymers, during or after assembly process, have not been well understood. Mixing phospholipids and block copolymers forming hybrid vesicle has been proposed to tune the membrane properties to combine the robustness and chemical tunability of polymersomes with the softness, fluidity and biocompatibility of liposomes. The self-assembly, phase behaviour, membrane properties, and parameters controlling these hybrid vesicles are not yet fully understood. In chapter 3, nanoscale hybrid phospholipid/block copolymer vesicles of various compositions were prepared by film-hydration-extrusion and the mixing behaviours, stability, and membrane properties of these nanostructures have been explored. It was found that the size, encapsulation efficiency and content-release behaviour of nanoscale polybutadiene-b-poly(ethylene oxide) (PB-PEO)/1-palmitoyl-2-oleoyl-sn-glycero-3-hosphatidylcholine (POPC) hybrid phospholipid/block copolymer vesicles can be tuned by the mixing ratio of the amphiphiles. It was generally believed that in binary mixtures of phospholipids and block copolymers, each forms spherical vesicles when present alone, and form spherical hybrid vesicles in aqueous solution. Detailed characterizations by Cryo-TEM reveal that, surprisingly, mixing POPC and PB-PEO results in spontaneous formation of tubular vesicles. This tendency for tubular vesicle formation is most pronounced when the amphiphile composition reaches equimolar proportion of the lipid and the polymer. The spontaneous stabilization of tubular vesicles, which requires a significant spontaneous bilayer curvature or energy, was proposed to arise from the unusually asymmetric lipid/polymer membrane composition. Membrane active polypeptides are of large interest for development of novel therapeutics including peptide-mediated drug delivery and release systems, transfection agents and antibiotics. Finding routes to tune specificity and activity of membrane active polypeptides is hence of large importance to improve their efficacy and minimize harmful side effects. Chapter 5 describes a de novo designed antimicrobial-like helix-loop-helix polypeptide (JR2KC) that partition into zwitterionic lipid membranes when specifically and covalently anchored to the membrane. Anchoring and subsequent membrane partitioning triggers a structural transition of the polypeptide from random coil to α-helical conformation and induces pore formation. The extent of pore formation can be dynamically tuned by varying the number of anchoring moieties or by introducing a complementary polypeptide (JR2E) that is designed to heterodimerize with JR2KC and fold into four-helix bundles. Dimerization competes with membrane partitioning of the polypeptides and reduces the pore forming capacity of JR2KC. This system thus enables a precise and very specific route for tuning the permeability of lipid membranes and demonstrates a novel strategy for development of recognition based membrane active polypeptides. In Chapter 6, a tetramethylrhodamine-labeled peptides binder was assembled on graphene oxide in solution to develop as a fluorescence turn-on sensor for Lipopolysaccharide (LPS)/endotoxin detection. The fluorescence of the dye-labeled peptide is quenched upon interaction with GO. Specific binding to LPS triggers release of the peptide-LPS complex from GO, resulting in fluorescence recovery. This fluorescent turn-on sensor offers a limit of detection of 130 pM, which is the lowest ever achieved among all synthetic LPS sensors to-date. Importantly, this sensor is applicable for detection of LPS in commonly used clinical injectable fluids, and it enables selective detection of LPS from different bacterial strains as well as LPS on the membrane of living E. coli. Finally, the applications of self-assembled membranes, liposomes, polymersomes, and polypeptides for biosensing will be also demonstrated in the latter part of the thesis.Doctor of Philosophy (MSE