Lipid chain length effect on the phase behaviour of PCs/PEG:2000-PEs mixtures. A spin label electron spin resonance and spectrophotometric study.

Spin-label electron spin resonance (ESR) spectroscopy and spectrophotometry at fixed wavelength are used to study fully hydrated aqueous dispersions of phosphatidylcholines (PCs) with poly(ethylene glycol:2000)-phosphatidylethanolamines (PEG:2000-PEs). PEG:2000-PE is a micelle-forming polymer-lipid that is extensively used for increasing the lifetime of PC liposomes in the blood circulation through a steric stabilisation effect. The PC lipids and the PEG:2000-PE polymer-lipids have the same acyl chain length of either dimiristoyl (DM) or distearoyl (DS) chains. DMPC/PEG:2000-DMPE and DSPC/PEG:2000-DSPE mixtures were investigated over the entire range of relative compositions (0-100 mol%). In both dispersions, the low-temperature conventional spin label ESR spectra and the temperature dependence of the absorbance at 400 nm give an indication of the conversion from lamellae to micelles with increasing PEG:2000-PEs content. The physical state of the lipid assemblies, lamellar or micellar, is dependent not only on PEG:2000-PEs content, but also on the length of hydrocarbon chain of the lipid matrix. Micellisation is attained more readily in dispersions with longer hydrocarbon chains (i.e. in DSPC/PEG:2000-DSPE mixtures) than in those with shorter acyl chains (i.e. in DMPC/PEG:2000-DMPE mixtures). Saturation transfer ESR (ST-ESR) and absorbance measurements reflect the disaggregation of the bilayers and a reduction in the size of the lipid aggregates by PEG:2000-PEs at low content.

Lipid membrane expansion and micelle formation by polymer-grafted lipids: scaling with polymer length studied by spin-label electron spin resonance.

Spin-label electron spin resonance (ESR) spectroscopy and auxiliary optical density measurements are used to study lipid dispersions of N-poly(ethylene glycol)-dipalmitoyl phosphatidylethanolamine (PEG:5000-DPPE) mixed with dipalmitoyl phosphatidylcholine (DPPC). PEG:5000-DPPE bears a large hydrophilic polymer headgroup (with approximately 114 oxyethylene monomers) and is commonly used for steric stabilization of liposomes used in drug delivery. Comparison is made with results from mixtures of DPPC with polymer lipids bearing shorter headgroups (approximately 45 and 8 oxyethylene monomers). ESR spectra of phosphatidylcholine spin-labeled on the 5-C atom position of the sn-2 chain are shown to reflect the area expansion of the lipid membranes by the lateral pressure exerted in the polymer brush, in a way that is consistent with theory. The lipid chain packing density at the onset of micelle formation is the same for all three PEG-lipids, although the mole fraction at which this occurs differs greatly. The mole fraction at onset scales inversely with the size of the polymer headgroup, where the experimental exponent of 0.7 is close to theoretical predictions (viz. 0.55-0.6). The mole fraction of PEG-lipid at completion of micelle formation is more weakly dependent on polymer size, which conforms with theoretical predictions. At high mole fractions of PEG:5000-DPPE the dependence of lipid packing density on mole fraction is multiphasic, which differs qualitatively from the monotonic decrease in packing density found with the shorter polymer lipids. Lipid spin-label ESR is an experimental tool that complements theoretical analysis using polymer models combined with the lipid equation of state.

Molecular and mesoscopic properties of hydrophilic polymer-grafted phospholipids mixed with phosphatidylcholine in aqueous dispersion: interaction of dipalmitoyl N-poly(ethylene glycol)phosphatidylethanolamine with dipalmitoylphosphatidylcholine studied by spectrophotometry and spin-label electron spin resonance.

Spin-label electron spin resonance (ESR) spectroscopy, together with optical density measurements, has been used to investigate, at both the molecular and supramolecular levels, the interactions of N-poly(ethylene glycol)-phosphatidylethanolamines (PEG-PE) with phosphatidylcholine (PC) in aqueous dispersions. PEG-PEs are micelle-forming hydrophilic polymer-grafted lipids that are used extensively for steric stabilization of PC liposomes to increase their lifetimes in the blood circulation. All lipids had dipalmitoyl (C16:0) chains, and the polymer polar group of the PEG-PE lipids had a mean molecular mass of either 350 or 2000 Da. PC/PEG-PE mixtures were investigated over the entire range of relative compositions. Spin-label ESR was used quantitatively to investigate bilayer-micelle conversion with increasing PEG-PE content by measurements at temperatures for which the bilayer membrane component of the mixture was in the gel phase. Both saturation transfer ESR and optical density measurements were used to obtain information on the dependence of lipid aggregate size on PEG-PE content. It is found that the stable state of lipid aggregation is strongly dependent not only on PEG-PE content but also on the size of the hydrophilic polar group. These biophysical properties may be used for optimized design of sterically stabilized liposomes.

Sterically stabilized liposomes of DPPC/DPPE-PEG:2000. A spin label ESR and spectrophotometric study.

The chain dynamics and the thermotropic phase behavior of sterically stabilized liposomes obtained introducing in the host bilayer matrix of DPPC up to 7 mol% of the polymer-lipid DPPE-PEG:2000 were investigated by spin label electron spin resonance spectroscopy and spectrophotometry. The experimental data indicate that the dispersions have the dynamic and thermotropic characteristics of normal lamellar phase. Moreover, using spin labels that locate both in the interfacial and in the hydrocarbon regions, namely TEMPO-stearate, 5- and 16-PCSL, we find that relative to the unmodified DPPC bilayers, the polymer-grafted bilayers are loosely packed in the interfacial region and have reduced chain mobility in the gel phase. From the temperature dependence of the partition coefficient (P(c)), of the spin probe DTBN between the aqueous and the fluid hydrophobic regions of the bilayers and from the melting curves of the absorbance at 400 nm, we observe a slight influence on the endothermic phase transitions when increasing the concentration of the polymer-lipid in the DPPC bilayers, the influence being more evident in the pre-transition.