The effects of PEO-PPO-PEO triblock copolymers, mainly Poloxamer 188, on phospholipid membrane integrity under osmotic gradients were explored using giant unilamellar vesicles (GUVs). Fluorescence leakage assays showed two opposing effects of P188 on the structural integrity of GUVs depending on the duration of their incubation time. A two-state transition mechanism of interaction between the triblock copolymers and the phospholipid membrane is proposed: an adsorption (I) and an insertion (II) state. While the triblock copolymer in state I acts to moderately retard the leakage, their insertion in state II perturbs the lipid packing, thus increasing the membrane permeability. Our results suggest that the biomedical application of PEO-PPO-PEO triblock copolymers, either as cell membrane resealing agents or as accelerators for drug delivery, is directed by the delicate balance between these two states.

Certain amphiphilic block copolymers are known to prevent aggregation of unfolded proteins. To better understand the mechanism of this effect, the optical properties of heat-denatured and dithiothreitol (DTT) reduced lysozyme were evaluated with respect to controls using UV-Vis spectroscopy, transmission electron microscopy (TEM) and circular dichroism (CD) measurements. Then, the effects of adding Polyethylene Glycol (8000 Da), the triblock surfactant Poloxamer 188 (P188), and the tetrablock copolymer Tetronic 1107 (T1107) to the lysozyme solution were compared. Overall, T1107 was found to be more effective than P188 in inhibiting aggregation, while PEG exhibited no efficacy. TEM imaging of heat-denatured and reduced lysozymes revealed spherical aggregates with on average 250–450 nm diameter. Using CD, more soluble lysozyme was recovered with T1107 than P188 with β-sheet secondary structure. The greater effectiveness of the larger T1107 in preventing aggregation of unfolded lysozyme than the smaller P188 and PEG points to steric hindrance at play; signifying the importance of size match between the hydrophobic region of denatured protein and that of amphiphilic copolymers. Thus, our results corroborate that certain multi-block copolymers are effective in preventing heat-induced aggregation of reduced lysozymes and future studies warrant more detailed focus on specific applications of these copolymers.

Disruption of cell membranes triggers rapid metabolic energy exhaustion, then acute cellular necrosis. Cell membrane dysfunction due to loss of structure integrity is the pathology of tissue death in trauma, muscular dystrophies, reperfusion injuries and common diseases. It is now established that certain PEG-based biocompatible polymers, such as Poloxamer 188, Poloxamine 1107 and PEG, are effective in sealing of injured cell membranes, and thus can prevent acute necrosis if delivered within a few hours after injury. Despite these broad applications of PEG-based polymers for human health, the fundamental mechanisms of how PEG-based polymers interact with cell membranes are still under debate. Here, the effects of PEG-based biocompatible polymers on phospholipid membrane integrity under external stimuli (osmotic stress and oxidative stress) were explored using giant unilamellar vesicles (GUVs) as model cell membranes. Through fluorescence leakage assays and time-lapse fluorescence microscopy, we directly observed that the surface-adsorbed P188 can efficiently inhibits the loss of structural integrity of giant unilamellar vesicles (GUVs) under hypo-osmotic stress. We propose that the adsorption of polymers on the membrane surface is responsible for the cell membrane resealing process, while the insertion of the hydrophobic portion of the polymers increases membrane permeability. To elucidate the mechanism by which hydrophilic polymers help restore membrane integrity while their hydrophobic counterparts disrupt it, 1H Overhauser Dynamic Nuclear Polarization (ODNP)-NMR spectroscopy, a newly developed NMR technique that provides unprecedented resolution for differentiating weak surface adsorption versus translocation of polymers to membranes, was employed to sensitively detect polymer-lipid membrane interactions through the modulation of local hydration dynamics in lipid membranes. Our study shows that P188—the most hydrophilic poloxamer known as a membrane sealant—weakly adsorbs onto the membrane surface, yet effectively retards membrane hydration dynamics. Contrarily, P181—the most hydrophobic poloxamer known as a membrane permeabilizer—initially penetrates past lipid headgroups and enhances intrabilayer water diffusivity. Consequently, our results illustrate that the relative hydrophilic/hydrophobic ratio of the polymer dictates its functions. These findings gleaned from local hydration dynamics are well supported by our thermodynamics and fluorescence data.

the effect of glycerol on the surfactant -lipid system in terms of surfactant selfassociation (critical micelle concentration, CMC), membrane partitioning (partition coefficient K and aHmic), and the onset of membrane solubilization (i.e., bilayer-to-micelle transition, characterized by a specific surfactant-to-lipid mole ratio in the membrane denoted Resat) by isothermal titration calorimetry (ITC). One effect expected for glycerol is its tendency to 'salt out' hydrophobic molecules. This promotes all aggregation phenomena (K increases, CMC decreases) but has little effect on the balance between the aggregates (Resat const.). Furthermore, glycerol gradually dehydrates the polar head group, which renders the effective molecular shape more favorable for a bilayer (K increases) whereas micellization is much less affected (CMC ~const) so that bilayers are stabilized compared to micelles (Resat increases). Our results indicate that the behavior of the sugar based surfactant octyl glucoside seems governed by the salting out effect, whereas for ethylene oxide surfactant C12EO8, headgroup dehydration seems to be the key effect explaining the effects of glycerol.

There is a long-standing debate on whether general anesthetics act through a non-specific perturbation of bilayer physical properties or through binding to specific sites within ion channels, particularly the GABA-A receptor. In this study, we demonstrate that a series of liquid n-alcohol general anesthetics lower phase transition temperatures in giant plasma membrane vesicles, which have previously been shown to sit close to a miscibility critical point. All n-alcohols depress critical temperatures (Tc) by 451 C when added to vesicles at their anesthetic dose. Current work is investigating if transition temperatures are also depressed when n-alcohols are added to synthetic vesicles with critical lipid compositions. We also performed simulations of simplified receptors embedded in a nearly-critical membrane. In this model, receptor channels can be in two distinct internal states (conducting or non-conducting), and the occupancy of these states is allosterically regulated by their local lipid environment as well as the availability of ligand. We show that model channels with dimensions comparable to that of GABA-A could have their conductance increased by 50% when Tc is lowered by 4 C in the limit of low ligand concentration. This is in good agreement with experimental observations of GABA-A channels in the presence of general anesthetics (compare to Figure 2b in [1]). Taken together, these findings suggest that general anesthetics can have dramatic effects on the internal states of membrane bound proteins without requiring that they directly bind to specific sites. Instead, we propose that anesthetics may act by lowering the critical temperature of the membrane which in turn allosterically regulates ion channel function. 1. N. Franks and W. Lieb, Molecular and cellular mechanisms of general anesthesia. Nature, 367, 607 (1994).