Molecular Characterization of Gel and Liquid-Crystalline Structures of Fully Hydrated POPC and POPE Bilayers

Abstract

Molecular dynamics simulations were used for a comprehensive study of the structural properties of monounsaturated POPC and POPE bilayers in the gel and liquid-crystalline state at a number of temperatures, ranging from 250 to 330 K. Though the chemical structures of POPC and POPE are largely similar (choline versus ethanolamine headgroup), their transformation processes from a gel to a liquid-crystalline state are contrasting. In the similarities, the lipid tails for both systems are tilted below the phase transition and become more random above the phase transition temperature. The average area per lipid and bilayer thickness were found less sensitive to phase transition changes as the unsaturated tails are able to buffer reordering of the bilayer structure, as observed from hysteresis loops in annealing simulations. For POPC, changes in the structural properties such as the lipid tail order parameter, hydrocarbon trans-gauche isomerization, lipid tail tilt-angle, and level of interdigitation identified a phase transition at about 270 K. For POPE, three temperature ranges were identified, in which the lower one (270-280 K) was associated with a pre-transition state and the higher (290-300 K) with the post-transition state. In the pre-transition state, there was a significant increase in the number of gauche arrangements formed along the lipid tails. Near the main transition (280-290 K), there was a lowering of the lipid order parameters and a disappearance of the tilted lipid arrangement. In the post-transition state, the carbon atoms along the lipid tails became less hindered as their density profiles showed uniform distributions. This study also demonstrates that atomistic simulations of current lipid force fields are capable of capturing the phase transition behavior of lipid bilayers, providing a rich set of molecular and structural information at and near the main transition state.

Molecular dynamics simulation of a mixed lipid emulsion model: Influence of the triglycerides on interfacial phospholipid organization

“…We simulated a hydrated POPC bilayer with the same parameters to compare our area per lipid results with the previously published results. The molecular area of our simulated POPC bilayer (63.9 Å 2 ) is close to that of a previously simulated hydrated POPC bilayer (%62 Å 2 ) [63]. These results are lower than the experimental published molecular area of a POPC bilayer (68.3 Å 2 ) [64], but of the same magnitude as our own molecular area measurements, obtained by monolayer compression on a Langmuir trough (61.0 Å 2 at 30 mN/m, the lateral pressure usually considered to be close to that in a biological membrane) ( Table 2).…”

Analysis of lipid surface area in protein–membrane systems combining voronoi tessellation and monte carlo integration methods

“…[33] In Figures 5A and 5B, the surface area of lipids in the vicinity of the protein is very small, which is ascribed to an overlap of protein atoms and lipids as mentioned above. In Figure 5C, the membrane thickness near the protein is increased or decreased to some extent [e.g., near the points (a)], suggesting hydrophobic mismatch between protein and lipid membrane.…”

Automated tracking and analysis of phospholipid vesicle contours in phase contrast microscopy images

“…The intensity of Brownian motion namely also increases with temperature, but only when observing liquid structures, therefore the observed fluctuations may be also affected by vesicle state transition. For example, the transition from gel to liquid-crystalline state of POPC was experimentally detected for temperatures between -3 and ?8°C [20]. The measurements for the 5°C temperature may be therefore related to such POPC state transition.…”