UC Davis

Lipids and amphiphilic block copolymers can be combined to form hybrid lipid/block copolymer membranes, which leverage both the biocompatibility of lipids and the tunability and mechanical stability of block copolymers. While lipid vesicles and nanoparticles have demonstrated clear success as drug delivery vehicles and hosts for reconstituted membrane proteins, the addition of block copolymers introduces the potential for further functionality, such as stimulus responsiveness and extended particle longevity. Multicomponent lipid biomembranes have been widely observed to phase separate into domains enriched in each component, and an understanding of the phase behavior of these membranes is crucial toward such applications. The presence of phase separation has been observed to impact the success of both membrane protein reconstitution and drug delivery. However, further work is necessary to investigate the phase behavior of hybrid lipid/block copolymer membranes. In this work, we explore phase separation phenomena in hybrid lipid/block copolymer membranes. First, we mapped a phase diagram for the phospholipid DPPC and polybutadiene-block-polyethylene oxide (PBdPEO950) using fluorescence spectroscopy. Analysis of trends in the temperature dependence of membrane fluidity and hydration allowed solid-to-fluid phase transition temperatures to be identified and an apparent phase diagram to be mapped. Fluorescence spectroscopy techniques were additionally extended to study phase separation phenomena in cholesterol-containing lipid POPC/egg sphingomyelin (SM) and hybrid PBdPEO/egg SM membranes. Temperature break-point (Tbreak) values for nanometer scale vesicles were determined by decomposition of emission spectra of the fluorescent probe laurdan into three lognormal curves. Tbreak values were in good agreement with the one- to two-phase transition temperature observed by microscopy for micron-scale giant unilamellar vesicles. Low miscibility of PBdPEO and egg SM were observed, as well as stronger partitioning of cholesterol into PBdPEO than into POPC. Finally, we investigated the effects of incorporating a carboxyl-terminated PBdPEO copolymer into hybrid vesicles. Tbreak values displayed dependence on pH, Debye length, and vesicle composition consistent with an electrostatic repulsion contribution to vesicle phase behavior. Additionally, at Debye lengths comparable to those at physiologically relevant ionic strength, Tbreak at pH 5.9 was observed to be slightly higher than at pH 7.0 for vesicles containing significant amounts of carboxyl-terminated PBdPEO. Such electrostatic effects may have the ability to drive membrane phase separation in response to pH drops—such as those observed after endocytosis—in physiologically relevant conditions, suggesting the utility of such materials for drug delivery.