Transient Ordered Domains in Single-Component Phospholipid Bilayers

We report evidence of dense, ordered nanodomains in single-component fluid lipid bilayers. Our atomic-scale molecular dynamics simulations suggest that the area available to a lipid acyl chain exhibits large fluctuations, resulting in denser and sparser domains. The sizes of the dense domains can be up to $\sim 10\mathrm{nm}$, and their lifetimes are of the order of $\sim 10\mathrm{ns}$. In addition, our simulations suggest that domains of lipids with highly ordered acyl chains form predominantly within the dense regions, their sizes ranging from a few chains up to a few nanometers, and with lifetimes between $\sim 10\mathrm{ps}–10\mathrm{ns}$. These observations shed light on the origin of experimentally observed fluctuations, as well as on the mechanisms of phase transitions in lipid membranes.

Figure 1

Visualization of a single configuration of 1152 molecule lipid bilayer together with probability distributions of order parameter and area per chain or head group. Panel (a) shows the area per tail in one monolayer, determined using a Voronoi tessellation of the c.m. coordinates of the tails. Panel (b) shows the average order parameter $S_{zz}$ for each tail in the same configuration; the extremum values of $S_{zz}$ are 1 and $-0.5$, corresponding to a perfectly ordered (all-trans) state and a conformation perpendicular to the membrane normal, respectively. Lack of order is characterized by $S_{zz}\approx 0$. Panels (c) and (d) show the undulations and the membrane thickness, respectively. Panels (e) and (f) show the probability distributions of the order parameters and the areas for the whole trajectory. The head group order parameter was defined as the average of the chain order parameters.

Figure 2

Bivariate probability distribution of area per tail $A_t$ and order parameter $S_{zz}$. The contours are on logarithmic scale.

Figure 3

Static structure factors calculated for 2D center-of-mass coordinates and averaged over monolayers. The inset shows the size distribution of ordered domains. The size was measured as the number of ordered chains ($S_{zz}>0.5$).