Modulation of intermembrane interaction and bending rigidity of biomembrane models via carbohydrates investigated by specular and off-specular neutron scattering

We designed artificial models of biological membranes by deposition of synthetic glycolipid membrane multilayers on planar silicon substrates. In contrast to commonly used phospholipid membranes, this offers the unique possibility to study the influence of membrane-bound saccharide chains (cell glycocalix) on the membrane mechanics. Taking advantage of the planar sample geometry, we carried out specular and off-specular neutron scattering experiments to identify out-of-plane and in-plane scattering vector components. By considering the effects of finite sample sizes, we were able to simulate the measured two-dimensional reciprocal space maps within the framework of smectic liquid-crystal theory. The results obtained both at controlled humidity and in bulk water clearly indicate that a subtle change in the molecular chemistry of the saccharides strongly influences intermembrane interactions and membrane bending rigidities.

Figure 1

(Color online) Chemical structures and space-filling models of the glycolipids used in this study. The Gent lipid (left) has a “bent” gentiobiose head group, while the Lac1 lipid (right) has a “cylindrical” lactose head group.

Figure 2

(Color online) Left: Geometry of the scattering experiments. The reciprocal space coordinates (i.e., the scattering vector components $q_z$ and $q_{\parallel }$) are functions of $\Gamma$ and $\Omega$ [Eqs. (1)]. Right: (main panel) Measured reciprocal space map from Gent-D lipid membranes at $80°\mathrm{C}$ and relative humidity $h_{\mathrm{rel}}\sim 95\%$, plotted as a function of $q_z$ and $q_{\parallel }$. The central vertical line corresponds to a specular condition $(\Gamma =2\Omega )$, while the horizontal stripes resulting from the sample periodicity are known as Bragg sheets. (Inset) Typical rocking scan around the second Bragg peak indicating the high alignment of the membrane multilayers to the substrate.

Figure 3

(Color online) Membrane displacement correlation functions $C_k\left(r\right)$, plotted for a given set of parameters: $\eta =0.04$, $\lambda =20\mathrm{\AA }$, $R=1\mu \mathrm{m}$. Inset: $g_0\left(r\right)$ for the same values of $\eta$ and $\lambda$, in the infinite case $(R=\infty )$, and in the finite case $(R=1\mu \mathrm{m})$, where the curve saturates to the finite value $2{\sigma }^2$.

Figure 4

(Color online) Scattering intensity of the second Bragg sheet as a function of the angles $\Omega$ and $\Gamma$. Left: experimental data; right: simulation. System: Gent-D in ${\mathrm{H}}_2\mathrm{O}$ atmosphere at $T=80°\mathrm{C}$ and $h_{\mathrm{rel}}\sim 95\%$.

Figure 5

(Color online) Force-distance curves of Gent-H (left) and Lac1-H (right), obtained from measurements of the lamellar periodicity at different relative humidities at $80°\mathrm{C}$. Two different regimes can be identified: Steric repulsion at higher pressures $(>\sim 5\times {10}^7\mathrm{Pa})$ and a second regime (see text for details) at lower pressures $(<\sim 5\times {10}^7\mathrm{Pa})$. The dashed and solid lines correspond to the models fitted to the data points within the two pressure regimes.

Figure 6

(Color online) Measured (data points) and simulated (solid line) second Bragg sheets of Gent-D (top) and Lac1-D (bottom) membrane multilayers at $80°\mathrm{C}$ and $h_{\mathrm{rel}}\sim 95\%$. Left column: Intensity integrated along $\Gamma$ plotted as a function of $q_{\Vert }$ (inset: log/log plot). Right column: width of the sheet along $\Gamma$ as a function of $q_{\Vert }$. Inset top right panel: Comparison of experiment (data points) and simulation (lines) for three different values of the cutoff radius $R$.

Figure 7

(Color online) Measured (data points) and simulated (solid line) second Bragg sheet of Gent-H membrane multilayers at $60°\mathrm{C}$ in bulk ${\mathrm{D}}_2\mathrm{O}$. Left: intensity integrated along $\Gamma$ plotted as a function of $q_{\Vert }$. Right: width of the sheet along $\Gamma$ as a function of $q_{\Vert }$.