Model Lipid Membranes on a Tunable Polymer Cushion

A hydrated, surface-tethered polymer network capable of fivefold change in thickness over a $25–37°\mathrm{C}$ temperature range has been demonstrated via neutron reflectivity and fluorescence microscopy to be a novel support for single lipid bilayers in a liquid environment. As the polymer swells from 170 to 900 Å, it promotes both in- and out-of-plane fluctuations of the supported membrane. The cushioned bilayer proved to be very robust, remaining structurally intact for 16 days and many temperature cycles. The promotion of membrane fluctuations offers far-reaching applications for this system as a surrogate biomembrane.

Figure 1

Poly(NIPAAm-co-MaBP)-membrane neutron reflectivity (NR) and scattering length density (SLD) profiles in collapsed and expanded states. The reflectivity data (a) and (c) are plotted using open shapes with error bars indicating 1 standard deviation. The solid lines are the best least-squares fits that correspond to the simplest models of physical relevance shown in panels (b) and (d). NR curves (a) show significant changes before and after deposition of a DPPC bilayer. The SLD profile (b) corresponds to the top reflectivity curve in (a) and depicts the collapsed polymer and membrane. At $25°\mathrm{C}$, the top NR fit (c) using a simple model is less precise owing to inhomogeneity of the expanded polymer. The emergence of fringes shows that the membrane remains intact after $5\times$ polymer expansion. The corresponding SLD profile (d) shows a significantly increased volume fraction of water near the membrane and increased out-of-plane membrane fluctuation amplitude. The in-plane fluctuation wavelength has been greatly exaggerated in the real-space cartoons of the polymer-membrane system (b) and (d). The NR curves (a) and (c) are offset for clarity.

Figure 2

Fluorescence microscopy image of DPPC containing 0.5 mol% Texas Red 1,2-Dihexanoyl-$sn$-Glycero-3-Phosphoethanolamine (TR-DHPE) bilayer on top of a poly(NIPAAm-co-MaBP) cushion at $25°\mathrm{C}$. The red areas are regions of higher TR-DHPE concentration. At $25°\mathrm{C}$, DHPE is fluid and preferentially phase segregates from the black DPPC gel domains.

Figure 3

The temperature and time dependence of the poly(NIPAAm-co-MaBP) cushion-bilayer system. Panel (a) shows SLD profiles for temperatures: 37, 34, 31, and $25°\mathrm{C}$. With decreasing temperature, the region occupied by the membrane (indicated by a shadow) increases its thickness and SLD. Panel (b) shows reflectivity curves measured at $37°\mathrm{C}$ before (black data) and after (grey data) several temperature cycles down to $25°\mathrm{C}$ over a period of 16 days. The solid line is the fit to the measurement after 16 days.