Structure and Orientational Texture of Self-Organizing Lipid Bilayers

The structure of single supported dipalmitoyl-phosphatidylcholine bilayers prepared by vesicle fusion or Langmuir-Blodgett-Schaeffer (LBS) deposition techniques was characterized by x-ray reflectivity and grazing incidence diffraction in bulk water. LBS bilayers display symmetric leaflets similar to monolayer structures, while vesicle fusion yields more inhomogeneous bilayers. Diffraction establishes that lipids are always coupled across the bilayer even when leaflets are deposited independently and suggests the existence of orientational texture.

Figure 1

(a) XR for a DPPC bilayer (DPPC45) deposited at $45\mathrm{mN}/\mathrm{m}$ at the quartz-${\mathrm{H}}_2\mathrm{O}$ interface by LBS and a DPPC monolayer at the air-water interface at a surface pressure of $40\mathrm{mN}/\mathrm{m}$. Symbols represent measured data and solid lines are fits corresponding to $\rho (z)$ shown in (b) and (c). The thick gray curve shows the family of fits within the selection criteria with ${\chi }^2$ values between 2.0 and 3.0. (b) $\rho (z)$ of DPPC bilayers. The gray curve demonstrates the LBS (DPPC45) bilayer structural variation over approximately a dozen measurements and four sample preparations. The dotted curves encompass the variation of vesicle fusion bilayers (DPPCves) and show a perturbation to the inner leaflet structure. The width of the dark “ribbon” corresponds to profiles generated from reflectivity fits plotted in (a), and depicts a measure of uncertainty in the realspace structure. 0 Å denotes the center of the bilayer. (c) Volume constrained model for the representative DPPC45 bilayer profile [dark ribbon in (b)]. The black line is the lipid contribution to the total electron density. The dark gray curve is $\rho (z)$ of a DPPC monolayer at $40\mathrm{mN}/\mathrm{m}$ corresponding to the monolayer reflectivity in (a).

Figure 2

(a) GIXD from DPPC monolayers at the air-water interface (top) and single bilayer membranes (bottom) at the quartz-water interface deposited by LBS (DPPC45, left) and vesicle fusion (DPPCves, right). The projection of the water surface (monolayers) and quartz substrate (bilayers) is defined as $q_z=0{\AA }^{-1}$. Two Bragg rods where observed. Miller indices, $\{(0,1)+(1,0)\}$ and $\{1,-1\}$, corresponding to a distorted hexagonal unit cell are equivalent to the centered-rectangular unit cell notation used in multilayer structures where $\{1,1{\}}_{\mathrm{rectangular}}$ corresponds to $\{(0,1),(1,0){\}}_{\mathrm{hexagonal}}$ and $\{0,2{\}}_{\mathrm{rectangular}}$ corresponds to $\{1,-1{\}}_{\mathrm{hexagonal}}$. For DPPCves, the curvature of the diffraction peaks along constant $q_{\mathrm{tot}}=(q_z^2+q_{xy}^2{)}^{1/2}$ is due to variability in the tilt angle normal to the interface of the diffracting lipids. (b) Bragg peaks from DPPC monolayers and bilayers. Solid lines are fits to the Bragg peaks using Voigt functions. (c) Bragg rods from DPPC monolayers and bilayers. Solid lines are fits to the models described in the text. The fit to the bilayer Bragg rod corresponds to the orientational texture model (described in the illustration).