Spatial Distribution of PEO–PPO–PEO Block Copolymer and PEO Homopolymer in Lipid Bilayers

Abstract

Maintaining the integrity of cell membranes is indispensable for cellular viability. Poloxamer 188 (P188), a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymer with a numberaverage molecular weight of 8700 g/mol and containing 80% by mass PEO, protects cell membranes from various external injuries and has the potential to be used as a therapeutic agent in diverse applications. The membrane protection mechanism associated with P188 is intimately connected with how this block copolymer interacts with the lipid bilayer, the main component of a cell membrane. Here, we report the distribution of P188 in a model lipid bilayer comprising 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) using neutron reflectivity (NR) and atomic force microscopy (AFM). We also investigated the association of a PEO homopolymer (PEO8.4K; M n = 8400 g/mol) that does not protect living cell membranes. These experiments were conducted following incubation of a 4.5 mmol/L polymer solution in a buffer that mimics physiological conditions with supported POPC bilayer membranes followed by washing with the aqueous medium. In contrast to previous reports, which dealt with P188 and PEO in salt-free solutions, both P188 and PEO8.4K penetrate into the inner portion of the lipid bilayer as revealed by NR, with approximately 30% by volume occupancy across the membrane without loss of bilayer structural integrity. These results indicate that PEO is the chemical moiety that principally drives P188 binding to bilayer membranes. No defects or phase-separated domains were observed in either P188-or PEO8.4K-incubated lipid bilayers when examined by AFM, indicating that polymer chains mingle homogeneously with lipid molecules in the bilayer. Remarkably, the breakthrough force required for penetration of the AFM tip through the bilayer membrane is unaffected by the presence of the large amount of P188 and PEO8.4K.

Manipulating Phospholipid Vesicles at the Nanoscale: A Transformation from Unilamellar to Multilamellar by an n-Alkyl-poly(ethylene oxide)

“…PEO chains when functionalized with hydrophobic molecules, such as pairs of dodecyl or cholesteryl ends, can anchor to the outer hydrophilic part of the bilayer, while chains form a “mushroom” or “brush” conformation that changes the rigidity of the bilayer . Extending to longer blocks such as PEO–PPO (poly­(propylene oxide)) diblock copolymers or PEO–PPO–PEO triblock copolymers, also known as poloxamers, introduce the opportunity to manipulate the liposomes from inside the bilayer and from the aqueous phase. ,, Poly­(oxyethylene alkyl ethers) (C i E j ) or n -alkyl PEO polymers are at an intermediate position between PEO and diblock copolymers such as PEO–PPO. These n -alkyl PEO polymers consisting of hydrophilic poly­(ethylene oxide) chains connected to simple hydrophobic alkyl chains are nonionic surfactant-like polymers that can show multiple self-assembled structures. − These are commonly used in soaps and detergents in combination with other surfactants.…”

Synthesis and Micellization of Bottlebrush Poloxamers

“…Poloxamers are a specific class of linear block polymer amphiphile of poly­(ethylene oxide) (PEO) and poly­(propylene oxide) (PPO) segments that are commercially available (e.g., Pluronics), are biocompatible, and have demonstrated efficacy as cell membrane stabilizing agents against many types of stresses. ,− A range of prior studies have focused on the mechanisms of poloxamer–lipid interactions; however, a complete picture of the stabilization mechanism is still lacking. ,,− ,− Amphiphilic bottlebrush polymers have potential as a tool to yield additional mechanistic insights, through architectural variation. Bottlebrush polymers have multiple parameters to tune (e.g., backbone length, graft length, graft spacing), and aspects of the relationships among these parameters and molecular properties, such as segmental dynamics, − molecular shape, and ability to share space, − are understood.…”

Lipid Membrane Binding and Cell Protection Efficacy of Poly(1,2-butylene oxide)-b-poly(ethylene oxide) Copolymers

“…However, PBO units could facilitate a deeper insertion depth for the polymer compared to PPO, but further studies are needed to confirm this. It should be noted that Kim, et al found that at high polymer concentration (≈40 mg/mL) P188 and PEO similarly permeated bilayers composed of POPC . Therefore, the PEO block could also play a part in facilitating membrane association.…”