After inhalation, lipopolysaccharide (LPS) molecules interfere with a pulmonary surfactant, a unique mixture of phospholipids (PLs) and specific proteins that decreases surface tension at the air–liquid interphase. We evaluated the behaviour of a clinically used modified porcine pulmonary surfactant (PSUR) in the presence of LPS in a dynamic system mimicking the respiratory cycle. Polymyxin B (PxB), a cyclic amphipathic antibiotic, is able to bind to LPS and to PSUR membranes. We investigated the effect of PxB on the surface properties of the PSUR/LPS system. Particular attention was paid to mechanisms underlying the structural changes in surface-reducing features. The function and structure of the porcine surfactant mixed with LPS and PxB were tested with a pulsating bubble surfactometer, optical microscopy, and small- and wide-angle X-ray scattering (SAXS/WAXS). Only 1% LPS (w/w to surfactant PLs) prevented the PSUR from reaching the necessary low surface tension during area compression. LPS bound to the lipid bilayer of PSUR and disturbed its lamellar structure by swelling. The structural changes were attributed to the surface charge unbalance of the lipid bilayers due to LPS insertion. PxB acts as an inhibitor of structural disarrangement induced by LPS and restores original lamellar packing, as detected by polarised light microscopy and SAXS.
pH-sensitive liposomes composed of homologues of series of N,N-dimethylalkane-1-amine N-oxides (CnNO, n = 8−18, where n is the number of carbon atoms in the alkyl substituent) and neutral phospholipid dioleoylphosphatidylethanolamine (DOPE) were prepared at two molar ratios (CnNO/DOPE = 0.4:1 and 1:1) and tested for their in vitro transfection activity. Several techniques (SAXS/ WAXS, UV−vis, zeta potential measurements, confocal microscopy) were applied to characterize the system in an effort to unravel the relationship among the transfection efficiency, structure, and composition of the lipoplexes. The transfection efficiency of CnNO/DOPE for plasmid DNA in U2OS cells follows a quasi-parabolic dependence on CnNO's alkyl substituent length with a maximum at n = 16. The transfection efficiency of CnNO/DOPE (n = 12−18) lipoplexes was found to be higher than that of commercially available Lipofectamine 2000. C16NO/DOPE also positively transfected HEK 293T and HeLa cells. Small-angle X-ray scattering (SAXS) shows large structural diversity depending on the complex's composition and pH. Transfection efficiencies mediated by two structures, either a condensed lamellar (L α c ) or epitaxially connected L α c and a condensed inverted hexagonal (H II c ) phase (L α c & H II c ), were found to be very similar. The change in pH from acidic to neutral induces phase transition L α c & H II c → Q II + L α , with cubic phase Q II of the Pn3m space group. Q II detected in lipoplexes of most efficient composition CnNO/DOPE (n = 16 and 18) facilitates DNA release and promotes its internalization in the cell.
Phospholipid-based non-viral carriers composed of neutral phospholipid dioleoylphosphatidylethanolamine (DOPE) and the binary mixture DOPE–oleic acid (OA) are examined as potential DNA delivery vectors. The process of DNA condensation in the presence of Ca2+ ions has been monitored through changes in emmision intensity of fluorescent probe ethidium bromide. The decline in fluorescence intensity with increasing Ca2+ concentration at two different time intervals was correlated with the binding capacity of complexes and possible release of DNA from the complex. The microstructure of DOPE–OA mixtures at different OA/DOPE molar ratios and that of DOPE–OA–DNA–Ca2+ complexes were determined using synchrotron small angle X-ray diffraction (SAXD). We identified inverted hexagonal phase HII as the dominant structure. OA affects the lattice parameter of HII formed by DOPE. With the increasing OA/DOPE molar ratio, the lattice parameter decreases, which results in significantly lower fraction of DNA bound to the OA-enriched complexes.
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