The effects of temperature, salinity, concentration and PEGylated lipid on the spontaneous nanostructures of bicellar mixtures

• DPPC/DHPC/DPPG mixtures have a disc-to-vesicle transition upon elevation of T.
• High lipid concentration, DPPG and PEGylated DSPE stabilize the nanodiscs.
• DPPG and PEGylated DSPE inhibit the formation of multi-lamellar vesicles.
• High salinity has opposite effects to those of DPPG and PEGylated DSPE.
• The nanodiscs of low-Clp samples are irreversible after disc-to-vesicle transition.

The self-assembling morphologies of low-concentration (mostly 1 and 10 mg/mL) bicellar mixtures composed of zwitterionic dipalmitoyl (di-C16) phosphatidylcholine (DPPC), dihexanoyl (di-C6) phosphatidylcholine (DHPC), and negatively charged dipalmitoyl (di-C16) phosphatidylglycerol (DPPG) were investigated using small angle neutron scattering, dynamic light scattering and transmission electron microscopy. A polyethylene glycol conjugated (PEGylated) lipid, distearoyl phosphoethanolamine-[methoxy (polyethyleneglycol)-2000] (PEG2000-DSPE), was incorporated in the system at 5 mol% of the total lipid composition. The effects of several parameters on the spontaneous structures were studied, including temperature, lipid concentration, salinity, and PEG2000-DSPE. In general, nanodiscs (bicelles) were observed at low temperatures (below the melting temperature, TM of DPPC) depending on the salinity of the solutions. Nanodisc-to-vesicle transition was found upon the elevation of temperature (above TM) in the cases of low lipid concentration in the absence of PEG2000-DSPE or high salinity. Both addition of PEG2000-DSPE and high lipid concentration stabilize the nanodiscs, preventing the formation of multilamellar vesicles, while high salinity promotes vesiculation and the formation of aggregation. This study suggests that the stability of such nanodiscs is presumably controlled by the electrostatic interactions, the steric effect induced by PEG2000-DSPE, and the amount of DHPC located at the disc rim.

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