Reversible Control of Spacing in Charged Lamellar Membrane Hydrogels by Hydrophobically Mediated Tethering with Symmetric and Asymmetric Double-End-Anchored Poly(ethylene glycol)s

Abstract

Complex materials often achieve their remarkable functional properties by hierarchical assembly of building blocks via competing and/or synergistic interactions. Here, we describe the properties of new double-end-anchored poly (ethylene glycol)s (DEA-PEGs), macromolecules designed to impart hydrophobically mediated tethering attractions between charged lipid membranes. We synthesized DEA-PEGs (MW 2000 (2K) and 4.6K) with two double-tail (symmetric) or a double-tail and a single-tail (asymmetric) hydrophobic end anchors and characterized their equilibrium and kinetic properties using small-angle X-ray scattering. Control multilayer membranes without and with PEG-lipid (i.e. single-end-anchored PEG) swelled continuously with the interlayer spacing increasing between 30wt% and 90wt% water content, due to electrostatic as well as, in the case of PEG-lipid, steric repulsion. In contrast, interlayer spacings in lamellar membrane hydrogels containing DEA-PEGs expanded over a limited water dilution range and reached a “locked” state, which displayed a near constant membrane wall-to-wall spacing (δw) with further increases in water content. Remarkably, the locked state displays a simple relation to the PEG radius of gyration δw ≈ 1.6 RG for both 2K and 4.6K PEG. Nevertheless, δw being considerably less than the physical size of PEG (2(5/3)1/2RG) is highly unexpected and implies that, compared to free PEG, anchoring of the PEG tether at both ends leads to a considerable distortion of the PEG conformation confined between layers. Significantly, the lamellar hydrogel may be designed to reversibly transition from a locked to an unlocked (membrane unbinding) state by variations in the DEA-PEG concentration controlling the strength of the interlayer attractions due to bridging conformations. The findings with DEA-PEGs have broad implications for hydrophobic-mediated assembly of lipid- or surfactant- coated building blocks with distinct shape and size, at predictable spacing, in aqueous environments.

Destabilization of the Nematic Phase of Clay Nanosheet Suspensions by Polymer Adsorption

“…Moreover, bridging conformations seem quite improbable because the average distance between nanosheets is at least 80 nm for all samples, and therefore much larger than the PEO end-to-end distance, resulting in an elastic energy penalty for stretching  k B T(d/R g ) 2 > 10 k B T. 40 This is consistent with the absence of any sign of gelation or polymer bridging flocculation in our samples. Therefore, in the following theoretical model (see next section), we consider that the PEO macromolecules adsorb as a succession of trains, loops, and tails, giving rise to a polymer coating layer on the beidellite nanosheets.…”

Assembly of Building Blocks by Double-End-Anchored Polymers in the Dilute Regime Mediated by Hydrophobic Interactions at Controlled Distances

“…We have previously prepared symmetric and asymmetric DEA-PEGs (Figure 2) from PEG of molecular weight 2000 and 4600 g/mol (n = 45 and 104), together with the corresponding SEA-PEGs. 50 As shown in Figure S1 in the Supporting Information, the symmetric DEA-PEGs (L-P2000-L and L-P4600-L) incorporate two dioleyloxybenzoic acid (DOB; double chain C18:1) tails, while the asymmetric DEA-PEGs (L′-P2000-L and L′-P4600-L) incorporate a DOB tail (L) and an oleic acid tail (L′; single chain C18:1). Single-and double-chain tails vary greatly in their water solubility (consider, e.g., the critical micelle concentration of lipids and surfactants, which differs by several orders of magnitude), which affects the stability of the intermediate conformation (Figure 2B).…”

“…We returned to membranes of low charge density (10% MVL5) to attempt to tune the DEA-PEG-induced interactions by introducing steric repulsion. To this end, we added a PEGlipid (L-P2000; i.e., an SEA-PEG) of identical PEG chain length to membranes containing 5 mol % of the L′-P2000-L that very effectively tethers membranes 50 (see also the sections below). As shown in Figure 5, increasing concentrations of added SEA-PEG led to an increasing dispersion of aggregates.…”

“…Finally, varying the structure of the second anchor allowed us to modify the kinetics of phase formation (by modulating the stability of the intermediate state in Figure 2B) independent of the overall anchoring strength (which is modulated by the mol fraction of DEA-PEG in the membrane as well as the structure of the anchor). 50 These studies provided proof-of-concept that the incorporation of DEA-PEGs can provide attractive, nonspecific interactions of tunable strength and range. However, the experiments used bulk lamellar phases with high lipid concentrations (≥10 wt % in water) and required equilibration times on the scale of weeks.…”

“…50 The resulting interlamellar distance is of the order of the radius of gyration (R G ) of the PEG tether: d W ≈ 1.6R G = 1.6an 3/5 , with a = 3.6 Å the monomer length and n the degree of polymerization of the PEG tether. 50 Thus, it is possible to adjust the wall-to-wall spacing (controlling the range of interaction) between apposing membranes by varying the length of the PEG tether. Furthermore, the strength of the tethering increased with increasing concentration of DEA-PEG in the lipid membrane.…”