Thin liquid films and monolayers of DMPC mixed with PEG and phospholipid linked PEG

Ethanol enhances collective dynamics of lipid membranes

From inelastic neutron-scattering experiments and all atom molecular dynamics simulations we present evidence for a low-energy dynamical mode in the fluid phase of a 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayer immersed in a 5% water/ethanol solution. In addition to the well-known phonon that shows a liquidlike dispersion with energies up to 4.5 meV, we observe an additional mode at smaller energies of 0.8 meV, which shows little or no dispersion. Both modes show transverse properties and might be related to molecular motion perpendicular to the bilayer.

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

In this work the capability of Progesterone (Prog) to penetrate to phosphatidyl serine (PS) monolayers (detected by equilibrium and dynamic surface tensions) and to induce rupture of PS thin liquid films (TLFs, known as foam films) in presence of Ca(2+) ions is studied. TLF studies reveal that the presence of Ca(2+) ions changes the type of PS films from thicker common black films to bilayer Newton black films and that the addition of Prog results in film destabilization and rupture. The effects of Prog in presence of Ca(2+) ions were observed with the films consisting of negatively charged PS but not of neutral phospholipids. The results correlate with the proposed physiological role of Prog and Ca(2+) in the acrosome reaction. The model of TLFs is used for the first time to study membrane fusion during acrosome reaction and proposes several new qualitative and quantitative parameters for studying of this reaction.

Effects of poly (ethylene glycol) chains conformational transition on the properties of mixed DMPC/DMPE-PEG thin liquid films and monolayers

Foam thin liquid films (TLF) and monolayers at the air-water interface formed by DMPC mixed with DMPE-bonded poly (ethylene glycol)s (DMPE-PEG(550), DMPE-PEG(2000) and DMPE-PEG(5000)) were obtained. The influence of both (i) PEG chain size (evaluated in terms of Mw) and mushroom-to-brush conformational transition and (ii) of the liposome/micelle ratio in the film-forming dispersions, on the interfacial properties of mixed DMPC/DMPE-PEG films was compared. Foam film studies demonstrated that DMPE-PEG addition to foam TLFs caused (i) delayed kinetics of film thinning and black spot expansion and (ii) film stabilization. At the mushroom-to-brush transition, due to steric repulsion increased DMPE-PEG films thickness reached 25 nm while pure DMPC films were only 8 nm thick Newton black films. It was possible to differentiate DMPE-PEG(2000/5000) from DMPE-PEG(550) by the ability to change foam TLF formation mechanism, which could be of great importance for "stealth" liposome design. Monolayer studies showed improved formation kinetics and equilibrium surface tension decrease for DMPE-PEG monolayers compared with DMPC pure films. SEM observations revealed "smoothing" and "sealing" of the defects in the solid-supported layer surface by DMPE-PEGs adsorption, which could explain DMPE-PEGs ability to stabilize TLFs and to decrease monolayer surface tension. All effects in monolayers, foam TLFs and solid-supported layers increased with the increase of PEG Mw and DMPE-PEG concentration. However, at the critical DMPE-PEG concentration (where mushroom-to-brush conformational transition occurred) maximal magnitude of the effects was reached, which only slightly changed at further DMPE-PEG content and micelle/liposome ratio increase.

Neuroprotection from secondary injury by polyethylene glycol requires its internalization

Polyethylene glycol (PEG) is well known to both fuse and repair cell membranes. This capability has been exploited for such diverse usages as the construction of hybridomas and as a reparative agent following neurotrauma. The latter development has proceeded through preclinical testing in cases of naturally induced paraplegia in dogs. The mechanisms of action of polymer-mediated neurorepair/neuroprotection are still under investigation. It is likely that the unique interaction of hydrophilic polymers with the mechanical properties of cell membranes in concert with an ability to interfere with mechanisms of secondary injury such as the production of highly reactive oxygen species (ROS or `free radicals') is the basis for neuroprotection by polymers.

Here we provide further evidence that the ability of PEG to reduce or limit secondary injury and/or lipid peroxidation (LPO) of membranes requires entry of PEG into the cytosol, further suggesting a physical interaction with the membranes of organelles such as mitochondria as the initial event leading to neurorepair/neuroprotection.

We have evaluated this relationship in vitro using acrolein, a potent endogenous toxin that is a product of LPO. Acrolein can pass through cell membranes with ease, inducing progressive LPO in `bystander' cells, and the production of even more acrolein by inducing its own production. Immediate application of PEG (10 mmol l–1, 2000 Da) to poisoned neurons in vitro was unable to rescue them from necrosis and death. Furthermore, three-dimensional confocal microscopy of fluorescently decorated PEG shows that it does not enter these cells for up to 2 h after application. By this time the mechanisms of necrosis are likely irreversible. Additionally,severe oxygen and or glucose deprivation of spinal cord white matter in vitro also initiates LPO. Addition of potent free radical scavengers such as ascorbic acid or superoxide dismutase (SOD) is able to interfere with this process, but PEG is not. Taken together, these data are consistent with the hypothesis that PEG is able to rescue mechanically damaged cells, based on a restructuring of the damaged plasmalemma. Furthermore, in compromised cells with an intact cell membrane, PEG must first gain access to the cytosol where this same capability may be useful in restoring the integrity of cellular organelles such as mitochondria, though the intracellular concentration of the polymer must be significant relative to the concentration of toxins produced by LPO in order to rescue the cell.

Preparation and characterization of pure and mixed monolayers of poly(ethylene glycol) brushes chemically adsorbed to silica surfaces

We prepared pure and mixed monolayers of methoxy-terminated poly(ethylene glycol)s (m-PEG's) chemically attached to silica surfaces by using m-PEG silane coupling agents of three different molecular weights. These films were subsequently characterized in water by atomic force microscopy (AFM). Images of pure m-PEG monolayers showed the formation of polymer brushes on silica. Force curves between two modified surfaces suggested that an increase in the number of oxyethylene (OE) groups from 6 (PEG6 surface) to 43 (PEG43 surface) to 113 (PEG113 surface) decreased the flexibility of the m-PEG chains in the m-PEG brushes. Frictional force measurements also showed that the friction increased in the order PEG6 < PEG43 <PEG113. Because PEG113 had a molecular weight that was greater than the critical molecular weight for entanglement in a PEG melt and displayed the least stretching of its chain, a chain in the PEG113 brush was thought to participate in entanglements or interchain hydrogen bonding. Mixed monolayers of PEG6 and PEG113 were prepared using various fractions of PEG6 and PEG113. Images of mixed PEG6 and PEG113 monolayers showed that the size of the PEG113 islands in the film decreased as the fraction of PEG113 decreased. The force curves between two modified surfaces suggested that the flexibility of the mixed monolayers decreased as the fraction of PEG113 increased. Frictional force measurements also showed that the friction decreased as the fraction of PEG6 in the PEG6-PEG113 mixed film increased. Entanglements were therefore thought to decrease as the fraction of PEG113 in the mixed monolayer decreased.