Cholesterol-Induced Modulated Phase in Phospholipid Membranes

We report the observation of a cholesterol-induced modulated phase ($P_{\beta }$) in dipalmitoyl phosphatidylcholine bilayers. It occurs below the main transition of the lipid at cholesterol concentrations of around 15 to 20 mol % and is distinct from the ripple ($P_{{\beta }^{\prime }}$) phase found in between the main and pretransitions at lower cholesterol concentrations. An electron density map of this phase, constructed from x-ray diffraction data from oriented multilayers, shows that the bilayers in this phase have a one-dimensional periodic height modulation with an amplitude of about 2.5 Å. A partial phase diagram of the system deduced from diffraction data is in broad agreement with earlier studies.

Figure 1

Partial phase diagram of DPPC-cholesterol mixtures obtained from the diffraction data. $P_{\beta }$ is the modulated phase induced by cholesterol. The precise locations of the phase boundaries have not been determined in this study.

Figure 2

The small angle diffraction pattern of the $P_{\beta }$ phase at $x_c=15$ and $T=6°\mathrm{C}$.

Figure 3

The wide angle chain reflection in the $P_{\beta }$ phase at $T=10°\mathrm{C}$. The scattered intensity is negligible for $q_z<0$ due to absorption by the substrate.

Figure 4

Electron density map of the $P_{\beta }$ phase of DPPC-cholesterol bilayers calculated from the diffraction data of Fig. 2. The solid (dotted) contours correspond to the electron rich (poor) parts of the bilayer. H, W, and C denote the head group, water, and chain regions, respectively. CH denotes the electron rich band in the bilayer due to the presence of cholesterol.

Figure 5

Profiles of the wide angle chain reflection in the different phases exhibited by the mixtures. The numbers against the curves indicate $x_c$. (a) and (b) refer to the on-axis ($q_z=0$) and off-axis reflections in the $L_{{\beta }^{\prime }}$ phase of DPPC, respectively. $x_c=20$ corresponds to the $P_{\beta }$ phase, and $x_c=33$ to the $L_{\alpha }$ phase.