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

Abstract

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.

Structural characterization of an elevated lipid bilayer obtained by stepwise functionalization of a self-assembled alkenyl silane film

“…Similar PEG-lipid micelle formation has been previously reported in the literature for a broad range of PEG-lipid molar concentration. 44,45 The region close to the interface ͑z =0−2 nm͒ exhibits a density, e , of about 400 e − /nm 3 , which is in good agreement with the presence of a PEG cushion of about 2 nm thickness.…”

“…The lack of such additional electron density in the profile of sample ͑4͒ indicates that in this case the lipid tails are embedded within the PEG region. These data clearly indicate that the backfilling of the PEG-lipid surface with PEG 45 causes the lipid tails to be expelled to the region above the PEG layer, which is likely favorable for the formation of an elevated lipid membrane as discussed in the next section.…”

“…3 with the reflectivity curves and corresponding electron density profiles for surfaces ͑5͒ and ͑6͒, which were obtained by the postfunctionalization of sample ͑4͒ with PEG 17 and PEG 45 , respectively. For reference, the profile of a bare silicon wafer with roughness of 0.47 nm has been superimposed on the electron density graph.…”

“…38 The radius of gyration of the polymer as formed in solution is estimated as a͑N /6͒ 1/2 with a being the unit length of PEG 45 ͑0.52 nm͒ and N the degree of polymerization ͑45͒. So for a PEG 45 polymer Rg is equal to 0.52͑45/ 6͒ 1/2 = 1.42 nm. If we allow for the SAM ͑up to 0.8 nm͒ and the lipid tails ͑up to 1 nm͒ the PEG layer thickness is at least 1.7 nm and therefore: z / Rg = 1.7/ 1.42= 1.2.…”

Influence of steroid hormone progesterone on the properties of phosphatidyl serine monolayers and thin liquid films

“…Progesterone is dissolved in chloroform, which is subsequently removed by at least 30 minutes evaporation under nitrogen stream and the sample is kept overnight under vacuum, as previously described [31]. The dry film obtained is hydrated at 70°C…”