Designing Size-Controllable Bicelles through Addition of Polyethylene Glycol Conjugated Lipid and Cholesterol

Our study focuses on effectively controlling the morphology of lipid-based nanodiscs (also known as “bicelles”). We have found that polyethylene glycol conjugated (PEGylated) lipid and cholesterol can enhance the stability, control the size of nanodiscs, and prevent a nanodisc to vesicle transition. Low incubation temperature can also improve the structural stability of the bicelles. Size-uniform bicelles were self-assembled in lipid mixtures composed of zwitterionic dipalmitoyl (di-C 16) phosphatidylcholine (DPPC), dihexanoyl (di-C6) phosphatidylcholine (DHPC), negatively charged dipalmitoyl (di-C 16) phosphatidylglycerol (DPPG) and sodium chloride. Small angle X-ray scattering, dynamic light scattering and transmission electron microscopy were used to characterize the nanostructure. We varied PEGylated lipid and cholesterol concentrations in the ranges of 0 - 5 mol% and 0 - 10 mol% of the total lipid composition, respectively. The samples with lower concentrations of PEGylated lipid (0%, 1%, 2.5%), and cholesterol (0%, 2%) showed substantial growth compared to the samples with higher concentrations. A structural transition from bicelles to vesicles was observed after the low PEGylated lipid and low cholesterol samples were incubated at 25 ℃ for several days, but was not present in samples containing higher amounts of cholesterol (5%, 10%) or PEGylated lipids (4%, 5%). Low temperature (4 ℃) significantly stabilizes the size of bicelles. It is possible to increase the size of bicelles at 25 ℃ until the desired size, then stabilize them by incubation at low temperature. These results have significant relevancy for pharmaceutical applications as it has been reported that nanodiscs have higher cellular uptake than vesicles. The ability to control the size of the nanodiscs would make it possible to optimize the system to allow for drug delivery to tumors with high efficacy.