la9b03208_si_001.pdf (2.68 MB)
Spatial Distribution of PEO–PPO–PEO Block Copolymer and PEO Homopolymer in Lipid Bilayers
journal contribution
posted on 2020-03-27, 21:29 authored by Mihee Kim, Frank Heinrich, Greg Haugstad, Guichuan Yu, Guangcui Yuan, Sushil K. Satija, Wenjia Zhang, Hannah S. Seo, Joseph M. Metzger, Samira M. Azarin, Timothy P. Lodge, Benjamin J. Hackel, Frank S. BatesMaintaining 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 number-average
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; Mn = 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.