Discontinuous Unbinding of Lipid Multibilayers

We have observed a discontinuous unbinding transition of lipid bilayer stacks composed of phosphatidylethanolamine and phosphatidylglycerol using x-ray diffraction. The unbinding is reversible and coincides with the main ($L_{\beta }\to L_{\alpha }$) transition of the lipid mixture. Interbilayer interaction potentials deduced from the diffraction data reveal that the bilayers in the $L_{\beta }$ phase are only weakly bound. The unbinding transition appears to be driven by an abrupt increase in steric repulsion resulting from increased thermal undulations of the bilayers upon entering the fluid $L_{\alpha }$ phase.

Figure 1

Unbinding transition upon heating through the main transition ($T_M=23.4°\mathrm{C}$) of a POPE$/$POPG ($87:13\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}$) mixture in $20\mathrm{m}M$ Na-phosphate buffer ($p\mathrm{H}=7.4$) containing $130\mathrm{m}M$ NaCl. The scattered intensity is plotted on a linear relative scale. Diffraction pattern (A), taken at $20°\mathrm{C}$, corresponds to a weakly bound multibilayer system. For clarity the position of the quasi-Bragg peaks are indicated by the dashes. Pattern (B) shows the system at $30°\mathrm{C}$, where interbilayer positional correlations are lost. The system returns to its loosely bound state after cooling back to $20°\mathrm{C}$ (C). Solid lines show full $q$-range fits (see text). The corresponding wide-angle diffraction patterns showing the in-plane order of the hydrocarbon chains are given in the inset.

Figure 2

Diffraction patterns of POPE$/$POPG ($87:13\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{m}\mathrm{o}\mathrm{l}$) bilayers in a $20\mathrm{m}M$ Na-phosphate buffer ($p\mathrm{H}=7.4$) in the absence of NaCl (a) and $800\mathrm{m}M$ NaCl (b). (i)–(iii) correspond to $20°\mathrm{C}$, $30°\mathrm{C}$, and $20°\mathrm{C}$, on cooling, respectively. At $20°\mathrm{C}$ the bilayers are in the $L_{\beta }$ phase and in the $L_{\alpha }$ phase at $30°\mathrm{C}$. Note that no quasi-Bragg peaks are seen in either of the two phases in the salt-free case (a), whereas, they are seen for both phases at $800\mathrm{m}M$ NaCl (b). The width of the peaks in the latter case is limited by the instrumental resolution, indicative of very long correlation lengths.

Figure 3

The total interaction potential as a function of bilayer separation $z$ for PE$/$PG bilayers at different NaCl concentrations: $800\mathrm{m}M$ (A), $130\mathrm{m}M$ (B), and $0\mathrm{m}M$ (C). The Hamaker constant $H$ was determined by the requirement that the minimum in (B) corresponds to the observed bilayer separation. There are no other unknown parameters in the expressions for the potential.