title: Formation of kinetically trapped nanoscopic unilamellar vesicles from metastable nanodiscs.
abstract: Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Importantly, in the case of these samples small angle neutron scattering (SANS) data show that, initially, nanodiscs (also known as bicelles) or bilayered ribbons form at low temperatures (i.e., 10 °C), but transform into uniform size, nanoscopic ULVs after incubation at 10 °C for 20 h, indicating that the nanodisc is a metastable structure. The instability of nanodiscs may be attributed to low membrane rigidity due to a reduced charge density and high salinity. Moreover, the uniform-sized ULVs persist even after being heated to 50 °C, where thermodynamically stable MLVs are observed. This result clearly demonstrates that these ULVs are kinetically trapped, and that the mechanical properties (e.g., bending rigidity) of 10 °C nanodiscs favor the formation of nanoscopic ULVs over that of MLVs. From a practical point of view, this method of forming uniform-sized ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on monodisperse lipid-based systems for therapeutic and diagnostic purposes.
authors.affiliation: Department of Chemical, Materials & Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, USA. mu-ping.nieh@ims.uconn.edu
authors.forename: Mu-Ping
authors.initials: MP
authors.lastname: Nieh
authors.forename: Paul
authors.initials: P
authors.lastname: Dolinar
authors.forename: Norbert
authors.initials: N
authors.lastname: Kučerka
authors.forename: Steven R
authors.initials: SR
authors.lastname: Kline
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Debeer-Schmitt
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Littrell
authors.forename: John
authors.initials: J
authors.lastname: Katsaras
mesh_terms: D007700:Kinetics; D053758:Nanoparticles / Q000737:chemistry*; D010743:Phospholipids / Q000737:chemistry; D013816:Thermodynamics; D053835:Unilamellar Liposomes / Q000737:chemistry*
chemical_list: D010743:Phospholipids; D053835:Unilamellar Liposomes